Explanation

(c) Protium$\ \left(_{1}^{1}H \right)$, deuterium$\left(_{1}^{2}H \right)$ and tritium$\left(_{1}^{3}H \right)$ are the isotopes of hydrogen. Out of the three, protium is most stable, deuterium is again stable but tritium is radioactive and eventually decays into an isotope of helium.