Which option is applicable to complete the following sentence?

The correct answer to this question is C.

As you move down an element group (column), the size of atoms increases. This is because each atom further down the column has more protons and neutrons and also gains an additional electron energy shell. Additionally, as you move across an element period (row), the overall size of atoms decreases slightly. Even though atoms further to the right have more protons, neutrons, and electrons, the outer electron shell is the same. The increased number of protons exerts a stronger positive charge, pulling the electrons in toward the nucleus.