In nuclear reactions, there is conservation of

The correct answer is Option (4) → mass, momentum and energy

Note: The given answer is as per the official NTA Answer Key. However, none of the given options appears to be correct, because rest mass is not conserved in a nuclear reaction; it seems that NTA has assumed “mass” to mean the mass number (or the total mass-energy). Strictly, mass-energy, momentum, charge and mass number are conserved, and no option states this, as explained below.

In every nuclear reaction the conserved quantities are the total energy (counting the rest-mass energy \( mc^{2} \), i.e. mass-energy), linear momentum, charge and the number of nucleons (mass number). The rest mass that disappears is released as energy, the Q value:

    \( Q = (m_{\mathrm{reactants}} - m_{\mathrm{products}})\,c^{2} \)

Option (1) mass only — Incorrect. Rest mass is not conserved.

Option (2) momentum only — Incorrect. Energy, charge and nucleon number are conserved as well.

Option (3) energy only — Incorrect. Energy is conserved, but it is not the only conserved quantity.

Option (4) mass, momentum and energy — Correct as per NTA; however, it is not correct as a general statement. It holds only if “mass” means the mass number or mass-energy.