Answer:
13H
Explanation:
All the other options listed are elements of the actinide series most of which are not naturally occurring. Tritium is a naturally occurring isotope of hydrogen believed to be present from the time of the formation of the earth. Due to its short half life of 12.26 years, any tritium present at that time must have decayed. However, it is constantly replenished by stellar reactions in the sun. Tritium is a beta emitter and also has uses in medicine.
