Which statement describes atomic radii as one moves from top to bottom within a group?
a. the atomic radius increases from top to bottom within a group.
b. the atomic radius decreases from top to bottom within a group.
c. the atomic radius does not vary in any predictable way within a group.
d. the atomic radius is the same within a group.

a.The atomic radii increases from top to bottom within a group.

Explanation:

We know that

When we move from top to bottom in  a group then the atomic radii increases When we move from top to bottom in a group then number of shell increases.

Therefore, the distance of electron in outermost shell from the nucleus is also increase.Hence, the atomic radii  increases.

But when we move from left to right in a group then nuclear pull increases because number of electron increases in the same shell.

Therefore, size of atom decreases and atomic radii decreases.

Hence, the atomic radii increases as one moves from top to bottom within a group.