Final answer:
The force that prevents two protons from combining in an atomic nucleus is the electrostatic force.
Step-by-step explanation:
In the context of atomic nuclei, the force that prevents two protons from combining is the electrostatic force. The electrostatic force is responsible for the repulsion between two particles of the same charge, like two protons. Since both protons have a positive charge, they repel each other. This force acts at a long range and is repulsive, as shown in Figure 10.5 (a) where the arrows represent outward forces on protons at the nuclear surface by a proton at the center. The other forces mentioned in the options - magnetic force, gravitational force, and nuclear force - are not the primary forces that prevent two protons from combining in this context.