Which element in the third period would you expect to have the larger atomic radius, sodium na or sulfur s?

                  Sodium (Na) has the larger Atomic Radius.

                           Remember there are three main factors which control or influence the atomic radius of elements.

i)  Electron Shells:

                             Greater the electron shells greater is the atomic radius and vice versa. As both Na ans S belongs to same group hence, this factor remains constant in both.

ii)  Nuclear Charge:

                               Greater the nuclear charge, smaller is the atomic radius and vice versa. As Na has less protons as compared to S, therefore its valence electrons will experience less Nuclear charge and will result in greater atomic radius as compared to S with more protons and greater Nuclear charge.

iii)  Shielding Effect:

                                The effect in which the core electrons shield the valence electron from nuclear attraction is know as Shielding effect. Greater the shielding effect greater is the atomic radius. In case of sodium and sulfur the shielding effect will remain same.

                   We can conclude that the size of Na is greater than Sulfur because of less Nuclear charge offered by sodium nucleus.

the answer your looking for is c).