The answer is Cl.
Group/family 18, the Nobel Gases, have full octets of 8 valence electrons. Each time you move to the LEFT one time within the nonmetals on the Periodic Table, an electron is lost. So, any element in group/family 17 would have 7 valence electrons (you can check this by drawing Bohr diagrams for each element within group/family 17).
So, we now know that the element is a nonmetal found in group/family 17. Therefore, it is found in the "p" block. For elements found in the "p" block, periods represent energy levels.