Which of the following isotopes of hydrogen is radioactive?

The correct answer is option 3,i.e. Tritium.

• Tritium is the radioactive isotopes of hydrogen.
• Hydrogen is a colourless, odourless, tasteless, flammable gasesous substance.
• A single hydrogen atom has one proton and one electron.
• One of two or more species of atoms of a chemical element with same atomic number but the different mass number is known as isotopes.  
• Three isotopes of hydrogen are found in nature:
  • Protium has no neutron.
  • Deuterium has 1 neutron.
  • Tritium has 2 neutrons.
• Deuterium is used to make heavy water(D2O).
• Heavy water used in nuclear reactors as a neutron moderator.