What is a group of protons and neutrons in the center of an atom called?

Ineed to find the answer of this question because i dont understand it

If a molecule with a molecular formula of c13h18 is treated with an excess of h2 in the presence of finally divided pt metal under conditions required for maximum hydrogenation of the molecule to give a molecule with a formula c13h24, how many rings are in the molecule?

If anyone would be able to me out with these three questions it would be these are from the chem 2202 course.