Question

as you move down a group, atomic radius increases because - you add more and more neutrons you add more and more shells (energy levels) you add more and more protons you add more atomic mass

Explanation:

As you move down a group in the periodic table, new electron - shells (energy levels) are added. Each new shell is farther from the nucleus, increasing the atomic radius. Neutrons do not directly affect the atomic radius in the same way as electron - shells. Adding more protons would increase the nuclear charge and tend to pull electrons in closer if not for the addition of new shells. Atomic mass is related to the number of protons, neutrons, but the key factor for atomic radius increase down a group is the addition of electron - shells.

Answer:

you add more and more shells (energy levels)