Define mass number protons plus the nucleus
protons plus electrons
electrons plus neutrons
protons plus neutrons

protons plus neutrons

Explanation:

An atom consist of electron, protons and neutrons. Protons and neutrons are present with in nucleus while the electrons are present out side the nucleus.

All these three subatomic particles construct an atom. A neutral atom have equal number of proton and electron. In other words we can say that negative and positive charges are equal in magnitude and cancel the each other. For example if neutral atom has 6 protons than it must have 6 electrons.

The sum of neutrons and protons is the mass number of an atom.

Mass number = number of protons + number of neutrons

while the number of protons or number of electrons is the atomic number of an atom.