The $Q$-value of a nuclear process is the difference between final and initial kinetic energies.
The nuclear mass is always higher than the total mass of its constituents.
Nuclides with same number of neutrons in the nucleus are known as isotopes.
In nuclear fusion, a heavy nucleus breaks into two smaller fragments.
Solution
Option (b) is incorrect as, nuclear mass is lower than mass of constituents due to mass defect.
Option (c) is incorrect as, Nuclides with same number of protons are isotopes.
option (d) is incorrect as, In fusion two or more lighter nuclei combines to form a heavy nucleus.
Option (a) is correct; as in a nuclear reaction $Q$
value $=K \cdot E_{\text {final }}-K \cdot E_{\text {initial }}$