Solution:
Option (iii) is the answer.
The concentration of tritium is approximately one atom per $10^18$ atoms of protium in the sample. Only tritium is radioactive and emits low-energy beta particles, making it the most dangerous of these isotopes.
Radioactive elements emit α, β and γ rays and are characterised by their halflives. The radioactive isotope of hydrogen is (i) Protium (ii) Deuterium (iii) Tritium (iv) Hydronium