The correct answer is option 4, which means that the assertion (A) is false, but the reason (R) is true.

Option 4 asserts that the electron affinity does not always increase from top to bottom in a group in the periodic table. This assertion is accurate as, generally, electron affinity decreases moving down a group (vertical column) on the periodic table. This trend occurs because electrons added to atoms further down a group must go into higher energy levels that are further from the nucleus, therefore, these atoms don`t attract additional electrons as strongly.

On the other hand, the reason (R) is true. In a group, atomic radii generally increase from top to bottom. This is due to an increase in the principal quantum number, i.e., the number of electron shells increases leading to a greater distance between the outermost electrons and the nucleus. This causes the atomic size (or atomic radii) to increase.

Option 4 is the correct answer since assertion (A) is false, and reason (R) is true.