Which of the following forms of hydrogen emits low energy $$\beta^-$$ particles?

We need to identify which form of hydrogen emits low energy $$\beta^-$$ particles.

Among the isotopes of hydrogen: Protium ($$_1^1H$$) has 1 proton and 0 neutrons, Deuterium ($$_1^2H$$) has 1 proton and 1 neutron, and Tritium ($$_1^3H$$) has 1 proton and 2 neutrons.

Protium and Deuterium are stable isotopes and are not radioactive. The proton $$H^+$$ is simply an ion and is also not radioactive.

Tritium ($$_1^3H$$) is the only radioactive isotope of hydrogen. It undergoes beta decay: $$_1^3H \to \, _2^3He + \beta^- + \bar{\nu}_e$$. The maximum energy of the emitted $$\beta^-$$ particles is about 18.6 keV, which is very low compared to beta particles from other radioactive nuclei.

The correct answer is Option (3): Tritium $$_1^3H$$.