Group 16 elements have a lower value of first ionization enthalpy as compared to group 15 elements. As group 15 elements have half filled p-orbital due to which group 15 have got extra stability.

VIA group or Group 16 elements belong to the oxygen family are also called chalcogens. The name is derived from the Greek word 'chalcos', which means 'ore' and 'gen', which means 'formation'. 

polonium has two metallic allotropes, the $\alpha $ atom has the simple cubic structure in a single atom basis at STP and $\beta $ atom of it has rhombohedral structure. Po belongs to group 16.

Ionisation energy decreases from top to bottom in a group. So increasing order will be
Po < Te < Se < S < O

Oxygen family is known as the chalcogen group.

Aluminum is the most abundant metal in the earth's crust. Oxygen is not a metal.

$\underset{VI group elements so belongs to chalcogens}{\underbrace{  8:oxygen   52:Tellurium   84:Polonium}} $

In VI group elements sulphur shows maximum no of valencies:-2,4,6

Boiling point is a physical property so depends on van der waal forces of attraction and van der waal forces $\propto$ molecular mass $\propto$ High boiling point
$\therefore $ Boiling point order :- $Te > Se > S > O$

Chalcogen is VI group (Oxygen family)
having elements:-
                 O
                  S
                  Se
                   Te
                    Po

$2KNO _3\overset{\Delta}{\rightarrow}2KNO _2+O _2\uparrow$

Priestley discovered oxygen gas in 1774. When he heated mercuric oxide, he found that a gas was released that could reignite a glowing splinter. This gas was oxygen.

The level of mercury in the bell jar rose by one-fifth during the experiment performed by Antonie Lavoisier.

During the experiment performed by Joseph Priestley, the observation was that a colourless gas is collected in the jar i.e oxygen.

Joseph Priestley did not use bell jar in his experiment.

During discovery of oxygen, mercuric oxide was heated by focusing the sun rays on it.

Antonie Lavoisier was the first scientist who named oxygen gas.

Oxygen has $6$ electrons in the outermost shell. Its configuration is $2,6$.

$16p$ means $Z=16$

$18e$ means $2$ unit negative charge is present.

Hence, species is $S^{2-}$.

Only one element of chromium from group $6$,that is $CrH$.

The atmospheric pollutants or gases that form smog are released in the air when fuels are burnt. When sunlight and its heat react with these gases and fine particles in the atmosphere, smog is formed. It is purely caused by air pollution. Ground level ozone and fine particles are released in the air due to complex photochemical reactions between volatile organic compounds (VOC), sulfur dioxide (SO2) and nitrogen oxides (NOx).

These VOC, SO2, and NOx are called precursors. The main sources of these precursors are pollutants released directly into the air by gasoline and diesel-run vehicles, industrial plants and activities, and heating due to human activities

As we move down the group, size increases and because of that M-M bond become weaker and Te-Te have least bond energy.

As oxygen is more electronegative so conjugate base in case of $H _2O$ is more stable and it is more basic than others.

All theses O,S,Se,Te,Po are group 16 elements from top to bottom.As we move from top to bottom in a group as the size increses the electronegativity  decreases.So most electronegative element is Oxygen
The order of electronegativity is O>S >Se>Te.Po.
Hence option A is correct.

The trend from nonmetallic to metallic character upon going down the group is exemplified by the conductivities of these elements. Sulphur is an insulator, selenium and tellurium are semiconductors.

The greater the element-element bond strength, the higher is the tendency towards catenation. 

Due to lower size and less electron-electron repulsion, S-S have highest bond energy.

Valency of Oxygen is 2

Mercuric chloride is red in colour.

Different combinations can be used in the preparation of water. Using these isotopes, a total of 18 combinations are possible. At first it might seem that kinds of water with the chemical structure $H _2O$ based on isotope differences only, are 9 in number:

$H^1 _2O^{16}, H^2 _2O^{16}, H^3 _2O^{16},$
$H^1 _2O^{17},H^2 _2O _{17},H^3 _2O^{17},$
$H^1 _2O^{17}, H^2 _2O^{18}, and H^3 _2O^{18}$

However, there are actually 18 differing isotope kinds of water. This is because each of the two hydrogen atoms in a particular molecule can be a different isotope, giving these 9 additional varieties of water:

$H^1H^2O^{16}, H^1H^3O^{16}, H^2H^3O^{16},$
$H^1H^2O^{17}, H^1H^3O^{17}, H^2H^3O^{17},$
$H^1H^2O^{18}, H^1H^3O^{18}, and H^2H^3O^{18}$

Usually, oxygen forms 2 bonds. So the covalency of oxygen is 2 as in $H _2O$. 

All the elements of oxygen family exist in more than one form (polymorphic).

Allotrope of oxygen which is known as ozone, is the primary absorber of UV radiation in arth's atmosphere.

All the elements in oxygen family are polymorphic (Polymorphism is the ability a material to exist in two or more forms or crystalline structures of the same chemical compound or element).
Hence option D is correct.

The long-term intake of excessive selenium may involve either organic or inorganic forms in food or water. Which affects the human health adversely. So it ispoisonous.

Hence option C is correct. 

Joseph Priestley discovered oxygen in 1774, he gave the answers why and how things burn.

Hence option A is correct.

In group $16$, the oxygen, sulphur, selenium are called chalcogens (ore-forming) while polonium being radioactive forms a less number of compounds and is not considered as chalcogens.

The molecular weights of the $N _2,O _2,CO _2,H _2$ are 28,32,44,2 respectively

In VI A group elements metallic properties increases down the group 
$\therefore $ O,S behave as typical non-metal.

$O _3$ is an allotrope of oxygen and hence, it belongs to VI A group.

Glass is made up of silicates, limestone is calcium carbonate. Ammonia does not contain oxygen. 

Nearly half the mass of earth's crust is oxygen in the form of silicates, limestone, metallic nitrates. 

Oxygen occurs in silicates and carbohydrates. 

It is a general test for the detection of halogens, nitrogen and sulphur in an organic compound. These elements are covalently bonded to the organic compounds. In order to detect them, these have to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. 

True.

Aluminium is the first most abundant metal while iron is the second most abundant metal on earth. It is the fourth most common element in the earth's crust. Oxygen is the most abundant non metal and second is silicon present on earth's crust.

The element name "oxygen" came from the greek words $oxys$ and $genes$, which together mean $"acid \ forming"$.

By mass, oxygen is the most abundant element in the human body.

Oxygen is found in the human body as oxy-haemoglobin in the bloodstream but not as $O _2$

Anions are of larger radius than neutral atom and smallest is cation
$O^{3-}  >  F^-  >  Li^+  >  Bi^{3+}$

The chalcogen means "ore former," derived from the Greek words chalcos ("ore") and -gen ("formation").