The increase in atomic radii in _ is due to an increase in the number of shells and the screening effect.

Explanation

As we move in a group from top to bottom, the number of shells and the effective nuclear charge increases but the effect of the addition of a new shell dominates over the effect of effective nuclear charge and hence the atomic size increases from top to bottom in a group.Hence, A is the correct option.As we move in a period from left to right, the number of shells remains the same as the electrons are added to the same shell. However, the effective nuclear charge increases and pulls the electron cloud closer to the nucleus. As a result, the atomic size decreases over a period.OPTION C: It is an incorrect option.OPTION D: It is an incorrect option.