The correct answer is D.
A nucleon
is one of either of the two types of subatomic particles (neutrons and protons) which are located in the nucleus of atoms.
The total number of nucleon in the nucleus of an atom gives you an idea about the mass of that atom. In fact, one may refer mass number as nucleon number.
Simply put, nucleons are the particles that make nucleus of an atom and are held up together inside the nucleus due to nuclear force.
