The answer would be:
D. Protons and neutrons
Here is more about the atomic model:
The atom can be divided into 2 areas. The nucleus and the electron cloud or orbit.
The nucleus is the found in the center of the atom, so like in your diagram, the center of the dartboard (Z). The nucleus makes up most of the mass in an atom and it contains two subatomic particles;
THE NUCLEUS PARTS:
The neutrons - neutral charged particles.
The protons - the positive charged particles.
Electrons on the other hand are found outside the nucleus.
