Protium, deuterium and tritium are the naturally occurring isotopes of:

Hydrogen

The correct answer is option 2. Hydrogen.

Protium, deuterium, and tritium are isotopes of hydrogen.

1. Protium:  This is the most common isotope of hydrogen and consists of one proton and one electron.

2. Deuterium:  Also known as heavy hydrogen, deuterium contains one proton and one neutron in its nucleus, making it twice as heavy as protium.

3. Tritium: Tritium is a radioactive isotope of hydrogen and contains one proton and two neutrons in its nucleus.

These isotopes of hydrogen differ in the number of neutrons in their nuclei while retaining the same number of protons, giving them similar chemical properties but different physical properties.