Which of the following is an example of nuclear fusion?

The correct answer is Option (3) → Formation of \( {}^{4}_{2}\mathrm{He} \) from \( {}^{2}_{1}\mathrm{H} \)

In nuclear fusion two light nuclei join to form a heavier nucleus, releasing energy because the product has a higher binding energy per nucleon. Two deuterium nuclei can fuse into a helium nucleus:

    \( \displaystyle {}^{2}_{1}\mathrm{H} + {}^{2}_{1}\mathrm{H} \rightarrow {}^{4}_{2}\mathrm{He} + \text{energy} \)

This is the kind of reaction that powers the sun and other stars.

Option (1) Formation of \( {}^{144}_{56}\mathrm{Ba} \) and \( {}^{89}_{36}\mathrm{Kr} \) from \( {}^{235}_{92}\mathrm{U} \) — Incorrect. A heavy uranium nucleus splitting into two middle-sized nuclei is nuclear fission.

Option (2) Formation of \( {}^{235}_{94}\mathrm{Pu} \) from \( {}^{235}_{92}\mathrm{U} \) — Incorrect. One heavy nucleus changes into another heavy nucleus; no light nuclei join, so this is not fusion.

Option (3) Formation of \( {}^{4}_{2}\mathrm{He} \) from \( {}^{2}_{1}\mathrm{H} \) — Correct. Light hydrogen nuclei join to make a heavier helium nucleus.

Option (4) Formation of water from hydrogen and oxygen — Incorrect. Forming water is a chemical reaction; atoms share electrons and the nuclei are unchanged.

NCERT (2026-27):
Nuclear Fusion – Energy Generation in Stars – “When two light nuclei fuse to form a larger nucleus, energy is released, since the larger nucleus is more tightly bound, as seen from the binding energy curve in Fig.13.1.”