Which of the following forms of hydrogen emits low energy β ^ -...

Tritium isotope of hydrogen is radioactive and emits low energy ${\beta}^{-}$ particles. It is because of high $n / p$ ratio of tritium which makes nucleus unstable.

For tritium \left(H 13\right)$</p><p>No. of neutron \left(n\right) = 2$

No. of proton \left(p\right) = 1$</p><p>$\dfrac{n}{p} = 2$</p><p>$∵ \dfrac{n}{p}$is high</p><p>tritium wil emit$\beta particle.