Choose the atom that has the smallest atomic radius. Na Li K Rb

Li has smallest atomic radius.

Explanation:

The given elements are present in group one. Among these Li has smallest radius.

Atomic radii trend along group:

As we move down the group atomic radii increased with increase of atomic number. The addition of electron in next level cause the atomic radii to increased. The hold of nucleus on valance shell become weaker because of shielding of electrons thus size of atom increased.

As the size of atom increases the ionization energy from top to bottom also  decreases because it becomes easier to remove the electron because of less nuclear attraction and as more electrons are added the outer electrons becomes more shielded and away from nucleus.

Li₃

Na₁₁

K₁₉

Rb₃₇

Thus, lithium has smallest atomic radius while rubidium has largest atomic radius.