What’s a element that is a non metal, contains 7 valance electron, 3 energy levels?

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. 

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current