Mendeleev left some blank spaces in the periodic table for those elements, which were not known at that time. He had predicted that these elements would be discovered later on. He even had predicted the properties of these elements in the light of other elements present in the same group. Mendeleev named these elements by prefixing the Sanskrit word ‘Eka’ to the name of preceding element in the same group, i.e. Eka–boron for scandium, Eka–aluminium for gallium and Eka–silicon for germanium.

Newlsnd's law of  octaves is applicable to elements with lower atomic masses only. Among the heavier elements the properties do not repeat after every eight elements.

Atomic number of elements is a more appropriate parameter to determine their properties. 

Ca, Ba and Sr are members of the same group. All the other sets contain elements from different groups.

Metallic character refers to the tendency to lose electrons. Li, Na, K and Rb are the members of group 1. As we move down the group from Li to Rb, the atomic size increases because the number of shells increases. Due to this increased atomic size, the effective nuclear charge experienced by electrons present in the outermost shell decreases. As a result, it becomes easier for an element to lose its electron, i.e. on moving down the group, metallic character increases. Therefore, Li is the least metallic, whereas Rb is the most metallic from all the given elements.

Mendeleev, in his periodic table, arranged the elements according to their increasing atomic masses but gave priority to the similarity in the fomulas of the oxides and hydrides while groupig them. As a result at some places, an element with a greater atomic mass was placed before an element with a lower atomic mass. For example, Co (atomic mass = 58.93 u) was placed before Ni (atomic mass = 58.71 u), which challenged the law that Mendleeev gave, that the properties of elements are periodic funtions of their atomic masses. Moreover the isotopes of an element have different atomic masses, but could not be alloted separate places in the periodic table on account of the similarity in the formulas their oxides and hydrides.  Therefore, (c) and (d) posed a challenge to Mendeleev's periodic law.

Option 4 is correct. Period (I) is the shortest period; it contains only two elements. Period (III) is short; it contains 8 elements. Period (V) is long period; it contains 18 elements. Period (VI) is the longest period; it contains 32 elements.

Newlands' law came to be known as law of octaves, not as law of triads.

All the statements given above were the discrepancies in Mendeleev's periodic table. 

As we move from top to bottom in a group, generally the atomic size of an element is increases. This is because the number of shells increase and so, the distance from nucleus increases.

As one moves across a period from left to right, the number of shells remains the same.

Since Mendeleev’s periodic law was based on atomic masses of the elements, the isotopes should have been  placed in different slots despite the fact that they represent the same element. This was one of the major disadvantage of Mendeleev’s periodic law.

On moving from left to right in a period, atomic radius decreases due to increase in the effective nuclear charge. Therefore the correct order in terms of increasing atomic radii is: Al > Si > P.

On moving left to right across a period, the atomic size decreases because the effective nuclear charge increases  

The elements of the triads had similar properties and when arranged in the increasing order of atomic masses, the atomic mass of the middle element was roughly the arithmetic mean of the masses of the other two elements.