Question

How many of the following properties show a clear periodic variation along a group?
(1) First ionisation energy
(2) Molar mass of the elements
(3) Number of isotopes of an element
(4) Atomic radius.

Answer 1

Molar mass of the elements show a clear periodic variation along a group.

First Ionization energy shows some irregularities and exceptions down a group in p-block. For example group 13.
There is no discernible trend for the periodic variation of the number of isotopes of an element along a group.
Atomic radius shows periodic variation along a group with certain irregularities.
Hence, the answer is $2$