What best describes the nucleus of a atom?

The best definition of a nucleus is: The central and most important part of an object, movement, or group, forming the basis for its activity and growth.So,

C. it is the part of the atom with the greatest mass

Explanation:

The nucleus of an atom is a small dense region at the center of an atom that contains protons and neutrons. Almost all the mass of an atom is located in the nucleus, with a very small contribution from the electron shells. The protons are positively charged which gives the nucleus a positive charge while the neutrons have no charge.

Hope this helps :)