The force that holds protons together in the nucleus of an atom is called the strong nuclear force.

Here's why:

* Protons are positively charged, and like charges repel each other.

* The strong nuclear force is much stronger than the electromagnetic force (which causes repulsion between protons).

* This force is extremely short-range, meaning it only acts over very small distances, effectively only within the nucleus.

The strong nuclear force is one of the four fundamental forces in nature, alongside gravity, the electromagnetic force, and the weak nuclear force. It's responsible for binding together protons and neutrons (which are neutral), forming the nucleus of an atom.