Some gases are taking three times as long to effuse out as helium.

The rate of effusion is always inversely proportional to the square root molecular mass.

The rate of effusion is the ratio of the volume effused to the time taken as follows.

$ \dfrac{{V} _{1}}{{t} _{1}} \times \dfrac {{t} _{2}}{{V} _{2}}$  $=\sqrt {\dfrac{{M} _{{w} _{2}}}{{M} _{{w} _{1}}}}$

 Volume is same, so

$\dfrac{3}{1}= \sqrt{\dfrac{{M _{w2}}}{4}}$

So, $M _{w2} =  36\ unit.$

The molecular mass of the gas is 36 u.

So, the correct option is A.

Two earlier group number systems exist: CAS (Chemical Abstracts Service) and old IUPAC. Both use numerals (Arabic or Roman) and letters A and B. In 1988 the new system simply numbers the groups increasingly from left to right on the standard periodic table. 

Mendeleevs Periodic Table:
Elements are arranged in the periodic table in the increasing order of their relative atomic masses.
Mendeleev divided his periodic table in eight groups and seven periods.
Groups from I to VII are meant for normal elements and group VIII is for transition elements.
Groups from I to VII have been divided in two sub groups, while group VIII is meant for three elements.
Periods from 4th to 7th have been divided in two series: 1st series and 2nd series.
Elements having similar properties have been kept in the same group. For example; lithium, potassium, rubidium, etc. are in 1st group.

Mendeleeve is credited with the development of his periodic table to illustrate periodic trends of known elements and predicted properties of unknown elements also.

Mendeleev’s Periodic table says that the periodic function of their relative atomic masses. Mendeleev arranged all the 63 elements in it. These only were discovered in his time. These are in the order of their increasing relative atomic masses in a tabular form. It is the Periodic Table. He had the table with eight columns and seven rows. The columns are the groups whereas rows are the periods.

Hence, the correct option is A.

After the rejection of Newlands Octave Law, Mendeleev Periodic table came into existence. In Mendeleev’s periodic table, elements were placed based on the atomic mass, and chemical properties. During Mendeleev’s work, only 63 elements were found. After studying the properties of all elements, he found that the properties of elements were related to atomic mass.

Hence, the correct option is C.

In 1869, Dmitri Mendeleev and Lothar Meyer individually came up with their own periodic law "when the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically." 

The elements selected by Mendeleev were oxygen and hydrogen because most of the elements reacted with oxygen and hydrogen.

The eight group in Mendeleev's periodic table was not divided into any group. It contained transition series elements.

The name given to the element, by Mendeleev, similar to Germanium in properties was Eka-Silicon.

The name given to the element, by Mendeleev, similar to Scandium in properties was Eka-Boron.

The groups from $1$st to $7$th of Mendeleev's periodic table were divided into two subgroups A and B.

The properties of Eka-aluminium are similar to gallium so it forms oxide of the type $E _2O _3$ same as $Ga _2O _3$.

The properties of Eka-aluminium are similar to gallium so it forms chloride of the type $ECl _3$ same as $GaCl _3$.

The elements which were unknown when Mendeleev gave his classification were geranium, gallium and scandium. He left gaps for the unknown elements and predicted correctly the properties of some of the unknown elements.

$4^{th}$, $5^{th}$ and $6^{th}$ period were divided into two series: first series and second series.

A Sanskrit numeral used before the names of the predicted elements was Eka which means one.

The elements having more masses were placed before elements having lower masses were cobalt, vanadium, tellurium. Its corresponding elements were nickel, chromium and iodine.

Mendeleev's Periodic Table was published in a German journal in 1872. 

There were many demerits in the Mendeleevs periodic table like Isotopes had to be given the same position in the periodic table though isotopes of the same elements have different atomic masses.

Mendeleev's periodic table failed because he couldn't tell about the position of isotopes, as he arranged elements on the basis of their increasing atomic mass.

According to Mendeleev's classification, the properties of elements are the periodic function of their atomic masses and there is a periodic recurrence of elements with similar physical and chemical properties. 

Eka-silicon is germanium, whereas eka-aluminium is gallium. Here germanium has 4 electrons in its outer most shell, hence it shares electron with chlorine, that too with 4. In case of gallium, it has three unpaired electrons in its outer most shell.

Professor Dmitri Mendeleev published the first periodic table of the atomic elements in 1869 based on properties which appeared with some regularity as he laid out the elements from lightest to heaviest. When Mendeleev proposed his periodic table, he noted gaps in the table, and predicted that as-of-yet unknown elements existed with properties appropriate to fill those gaps.

In 1869, just five years after Newlands put forward his Law of Octaves, a Russian chemist called Dmitri Mendeleev published a periodic table. Mendeleev also arranged the elements known at the time in order of relative atomic mass, but he did some other things that made his table much more successful. Hence he arranged according to atomic masses and their chemical properties.

In the Mendeleev's periodic table, the noble gases were included in the zeroth group.

Dmitri Mendeleev is credited with designing the modem periodic table. Joseph Priestly and Antoine Lavoisier were both chemists. Albert Einstein developed theories on photoelectric effect.

Isotopes could not find a position in Mendeleev's periodic table because those were arranged according to atomic number in the increasing order but isotopes have the same atomic number, all isotopes cannot be fitted in a single box in the periodic table. Hence, given statement is false.

In certain pair of elements, the increasing order of atomic masses was not obeyed. 

$Eka$-aluminium is known as $Gallium$.

Cycloalkanes are isomeric with alkenes because they have the same general formula CnH2n but possessing different structures, shows ring chain isomerism. Example- cyclohexane $(C _6H _12)$ and hexene $(C _6H _12)$ are ring chain isomers.

1st element of a group, have smaller size and high IP, also they dont have d-orbitals so they show different properties like valency, ionic character, maximum valency solubility.

According to Mendeleev's law, the physical and chemical properties of element are the periodic function of their atomic weights . (atomic masses)

Mendeleev periodic table is based on atomic weights of elements,

a.)The discovery of inert gases, later on, did not disturb Mendeleev's arrangement as he made quite clear that the noble gas must reside in the periods of inert gases having their octet i.e.completely filled electrons in their valence shell.

Mendeleev's law
"the statement that the chemical and physical properties of the elements recur periodically when the elements are arranged in the order of their atomic weights."

According to Mendeleev's periodic law physical and chemical properties both of the elements are the periodic function of their atomic masses

The noble gases were unknown to Mendeleev. They were discovered later.

In Mendeleev's periodic table, gap was not left for beryllium.

In 1870 Mendeleev created a table or chart that listed the known elements according to increasing order of atomic weights and when he organized the table into horizontal rows elements with similar chemical properties appeared at regular intervals.

Mendeleeve predicted several new elements and accurately predicted the qualities of missing elements like germanium,gallium and scandium. He arranged the elements in the table according to their atomic weight.

Out of the given options, Oxygen, Silicon and Chlorine were present. He also left gaps of elements to be discovered in future. One such gap was for germanium (Ge).

Eka- Aluminium (E) forms oxide with the same empirical formula is Aluminium oxide. Thus, it forms ${ E } _{ 2 }{ O } _{ 3 }$ oxide.

Period number 1, 2 and 3 contain 2, 8 and 8 elements respectively. Other periods contain more than 8 elements. Hence, the first three periods are called as short periods. So, total of 3 short periods are there in Mendeleev periodic table.

In the modified Mendeleev's periodic table, 8 groups and 7 periods are present, whereas in modern periodic table, 18 groups and 7 periods are present, respectively.

Cobalt was placed before nickel despite having atomic mass greater than that of nickel to maintain the periodicity in chemical properties.

Position of hydrogen was not right in the Mendleevs' periodic table. It was placed in IA. However, it resembles the elements of both IA(alkali metals) and group VII A (halogens).

Since, Mendeleev's classification is based on atomic weights, position of isotopes was not given. Lanthanides and Actinides were not known at that time. Their position has not been defined in the periodic table.

Fourteen elements that follow lanthanum called lanthanides and fourteen elements following actinium called actinides were not given proper places in Modern  periodic table.

The noble gases are chemically stable and hence usually very unreactive. Thus their properties are very difficult to observe. Hence Mendeleev was unable to predict their existence. Option B

The position of hydrogen was not justified in Mendeleev's periodic table. He positioned hydrogen in the first column above alkali metal. Hydrogen resembles not only alkali metals but also halogens in several ways. Hence, hydrogen can also be placed above the halogen group.

The discovery of isotopes was one of the limitations of Mendeleev's periodic table. Since Mendeleev's periodic table was based on atomic masses of elements, isotopes should be placed in different columns despite the fact that they represent the same element.

Mendeleev arranged the known elements in an increasing order of their atomic masses. According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic masses.

Chlorine, oxygen and silicon were included in Mendeleev's periodic table. Germanium was discovered later which fit into the empty spaces left by Mendeleev and matched to the expected properties. 

Eka-silicon means germanium. It occurred because of silicon whereas eka-aluminium occurred because of aluminium and later named it as gallium. 

According to Mendeleevs classification, the properties of elements are the periodic function of their atomic masses and there is a periodic recurrence of elements with similar physical and chemical properties. Noble gas being inert, could be placed in a separate group without disturbing the original order.

Mendeleev kept elements in increasing order of atomic weights in seven periods and eight groups.
Therefore, option $B$ is correct. 

Mendeleev described the elements according to both atomic weight and valency. 

Configuration of Cu and Ag is $\left[ Ar \right] 3{ d }^{ 10 }4{ s }^{ 1 }$ and $\left[ Kr \right] 4{ d }^{ 10 }5{ s }^{ 1 }$ respectively. Thus both have $1$ valence electron which is lost in cation formation. Thus correct answer is option B.

Answer A

He grouped the elements with similar properties and observed that elements were arranged in the increasing order of atomic mass and there was the periodic occurrence of elements with similar properties.

According to this observation, he formulated a periodic law which states:

“the properties of elements are the periodic function of their atomic masses.”

Option B is correct.

Mendeleev arranged the elements having similar configuration placed in the vertical columns as they will have the same unpaired electrons hence reflecting the similar properties. In Mendeleev's periodic table few elements that are chemically similar are placed in separate groups.

The horizontal rows in Mendleev's table were called periods. There are seven periods in Mendeleev's periodic table.

The vertical columns in Mendeleev's table were called groups. There are $8$ groups in Mendeleev's periodic table.

Newlands arranged elements in increasing order of their atomic weight and stated that every eighth element resembled properties of first element. This is Newland's law of Octaves. Hence correct answer for asked question is option C-atomic weight.

Dobereiner's triads are a group of three elements in which the atomic mass of the middle element is equal to the average of atomic masses of the first and the third element.

The first attempt to classify elements systemically was made by Dobereiner.
The triads of elements that showed similar chemical properties.
Other properties of the elements followed trends in which the value of the middle element of the triad was in between those of the end members.

Dobereiner classified the elements into triads (group of three). He made the groups such that, the mass of middle element is the average of the mass of other $2$ elements. This is Dobernier's law of triads.