Question

repulsion between closely packed protons in the nucleus is overcome by
gravitational forces
nuclear forces
magnetic forces
electrostatic forces

Explanation:

Protons in the nucleus are positively - charged and repel each other electrostatically. The strong nuclear force is a short - range force that overcomes this electrostatic repulsion and holds the nucleus together. Gravitational forces are too weak at the nuclear scale, magnetic forces are not relevant here, and electrostatic forces are the ones causing repulsion, not overcoming it.

Answer:

B. nuclear forces