Final answer:
The proton number of the atom before it loses electrons is 12.
Step-by-step explanation:
The atom with an atomic number of 12 that loses 2 electrons to become stable is a Magnesium atom. A neutral Magnesium atom has 12 protons, 12 electrons, and 12 neutrons. So the proton number of the atom before it loses electrons is 12, option c).