Which force is most responsible for binding together an atom's protons and neutrons? electrostatic gravitational nuclear magnetic

The answer is nuclear.

Protons have a positive charge and it is expected they will repel each other. Neutrons are without the charge. Electromagnetic forces are the cause why like charges repel and vice versa, why unlike charges attract. For an atom's protons and neutrons to hold together and not to fly apart, there must be a force that is stronger than the electromagnetic force. Therefore, the strong nuclear force between an atom’s protons and neutrons holds together the atom’s nucleus.