The correct answer is option 3, tritium.

Let`s break down the options to understand why tritium is the correct choice:

1. Protium: Protium is the most common isotope of hydrogen, accounting for about 99.98% of all hydrogen atoms. It does not have any radioactive properties. Therefore, it is not the correct answer.

2. Deuterium: Deuterium is another isotope of hydrogen which contains one neutron in addition to the usual proton. It is stable and not radioactive. Hence, it is not the correct answer.

3. Tritium: Tritium is the radioactive isotope of hydrogen. It contains two neutrons and one proton in its nucleus. It is produced in nature through cosmic ray interactions and can also be artificially produced. Tritium undergoes radioactive decay, emitting beta particles. It has a half-life of about 12.3 years. Therefore, tritium is the correct answer.

4. Hydronium: Hydronium is not an isotope of hydrogen but rather a polyatomic ion composed of three hydrogen atoms and one oxygen atom. It is not radioactive and does not fit the definition of a radioactive isotope of hydrogen. Therefore, it is not the correct answer.