p longrightarrow n + nu + A In this nuclear reaction A is a/an:

The correct answer is Option (3) → positron

A nuclear reaction must conserve electric charge (and the number of nucleons). The proton has charge \( +e \), while the neutron and the neutrino \( \nu \) are neutral.

    \( \displaystyle +e = 0 + 0 + q_A \;\Rightarrow\; q_A = +e \)

So A is a light particle with charge \( +e \): the positron. This is \( \beta^{+} \) decay, in which a proton changes into a neutron, emitting a positron and a neutrino.

Option (1) electron — Incorrect. An electron has charge \( -e \), so charge would not be conserved; the electron comes with an antineutrino in \( \beta^{-} \) decay of a neutron.

Option (2) gamma ray — Incorrect. A gamma ray has no charge, so the \( +e \) of the proton would be lost.

Option (3) positron — Correct. The positron carries charge \( +e \) that the proton had, and it is emitted together with a neutrino in \( \beta^{+} \) decay.

Option (4) anti neutrino — Incorrect. An antineutrino is neutral, and in \( \beta^{+} \) decay the particle emitted with the positron is a neutrino, not an antineutrino.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Beta Decay – “… In beta-minus decay, a neutron transforms into a proton within the nucleus according to

    \( \displaystyle n \rightarrow p + e^{-} + \bar{\nu} \qquad (13.24) \)

whereas in beta-plus decay, a proton transforms into neutron (inside the nucleus) via

    \( \displaystyle p \rightarrow n + e^{+} + \nu \qquad (13.25) \)”