The correct answer is: B. Exchange forces
Exchange forces are the fundamental forces that hold the protons and neutrons together in the nucleus of an atom. These forces are mediated by gluons, which are massless particles that carry the strong force. The strong force is much stronger than the electromagnetic force, which is why the protons and neutrons are able to stay together despite their positive charges.
Gravitational forces are much weaker than the strong force, and they would not be able to hold the protons and neutrons together in the nucleus. Coulombic forces are the electrical forces between the protons, which are positively charged particles. These forces would tend to push the protons apart, and they would not be able to hold the nucleus together. Magnetic forces are the forces between the protons and neutrons due to their magnetic moments. These forces are also very weak, and they would not be able to hold the nucleus together.
In conclusion, the correct answer is: B. Exchange forces.