Tritium is an isotope of :

The correct answer is (a). Tritium is an isotope of hydrogen. It has one proton, one neutron, and one electron. It is radioactive and decays into helium-3 with a half-life of 12.3 years.

(b) Carbon has six protons, six neutrons, and six electrons.
(c) Nitrogen has seven protons, seven neutrons, and seven electrons.
(d) Chlorine has 17 protons, 17 neutrons, and 17 electrons.