In nuclear fusion, once the two light nuclei come within the range of…

The correct answer is Option (4) → The nuclear forces dominate the electrostatic forces.

Two light nuclei repel each other electrostatically. So they need enough energy to overcome this Coulomb barrier. Only then can they come close enough for the short-range nuclear force to act on them.

Option (1) The nuclear forces become unimportant. — Incorrect. Within its range the attractive nuclear force is the force that pulls the two nuclei together into one nucleus.

Option (2) Only electrostatic forces act between them. — Incorrect. The nuclear force acts between nucleons only within a few femtometres. Once the nuclei are this close, it acts together with the Coulomb repulsion.

Option (3) The nuclear forces are dominated by the electrostatic forces. — Incorrect. If the Coulomb repulsion were stronger, the nuclei would be pushed apart again, and no nucleus could hold its protons together.

Option (4) The nuclear forces dominate the electrostatic forces. — Correct. The nuclear force is much stronger than the Coulomb force. So once the nuclei are within its range, the attraction is greater than the repulsion and the nuclei fuse.

NCERT (2026-27):
Nuclear Force – “(i) The nuclear force is much stronger than the Coulomb force acting between charges or the gravitational forces between masses. The nuclear binding force has to dominate over the Coulomb repulsive force between protons inside the nucleus.”

Nuclear Fusion – Energy Generation in Stars – “… For fusion to take place, the two nuclei must come close enough so that attractive short-range nuclear force is able to affect them. However, since they are both positively charged particles, they experience coulomb repulsion.”