Assertion (A) : In the periodic table of chemical elements, electron affinity is always found to increase from top to bottom in a group. Reason (R) : In a group, the atomic radial generally increase from top to bottom.

The Assertion is false: electron affinity does not increase down a group; it generally decreases (becomes less exothermic) from top to bottom in a group [1]. The Reason is true: atomic radius does increase down a group because electrons occupy higher shells and shielding increases. These two facts make A false and R true. Moreover, the increase in atomic radius and shielding (R) is actually a mechanistic reason for the observed decrease of electron affinity down a group (i.e., R would predict a lower, not higher, tendency to gain an electron), so R does not make A correct. Therefore the correct choice is 4 (A false, R true).

Sources

1. [1] https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.19%3A_Periodic_Trends_-_Electron_Affinity