only its protons only its electrons both its protons and neutrons both its neutrons and electrons
I gave my students this multiple choice question: A student needs to find the mass of an atom. The numbers of which atomic particles determines the mass of the atom?

Only one answer is correct among these 4 choices:
- only its protons
- only its electrons
- both its protons and neutrons
- both its neutrons and electrons

Could you tell me which one is correct?

both its protons and neutrons