The atomic number of magnesium is 12. The number of protons in the nucleus of magnesium atom is:

The proton, neutron, and electron are the three subatomic particles that make up an atom, which is the smallest unit of matter. The positively charged particles in the nucleus are referred to as protons, the negatively charged particles are referred to as electrons, and the neutrally charged particles, along with the protons, are referred to as neutrons.

An element is represented as:

              ${}^{\mathrm{A}}{\mathrm{X}}_{\mathrm{Z}}$

Where, X is the chemical symbol of the element.

 Z is the atomic number of an element which is equal to the number of protons of the neutral atom of the element. 

A is the mass number which is defined as the sum of the protons and neutrons of the element.

Now, the atomic number of magnesium (Mg) is 12.

Thus, the number of protons in the nucleus of the magnesium atom is 12.

Hence, option C is correct.