What is the force that holds protons together in the nucleus? (They should repel each other since they are positively charged)

The force is the strong nuclear force.

Explanation:

Every atom has a nucleus and various orbits surrounding it. The protons and neutrons are present in the nucleus while the electrons orbit around the nucleus. Protons are held together in the nucleus by the strong nuclear force. It is also called as the strong force. It holds together the subatomic particles within the nucleus. The nuclear force is the strongest of the four fundamental forces. Although it is the strongest, it can only act over short distances which are nuclear in length.

Letter c i’d say