Which of these elements is rarely found in organic molecules?

Hydrogen

Oxygen

Barium

Carbon

I'd say Barium. It's an alkaline earth metal, so it wouldn't normally be found in organic molecules.

Barium.

Explanation:

The important element present in the living organism is carbon, hydrogen, nitrogen and oxygen. These elements are important for the formation of biomolecules of the living organism.

Barium is rarely found in organic molecules as barium is the alkaline earth metal. The barium is generally found in plants and rarely present in the living organism. Barium does not play such an important role as played by the other organic molecule. Hence, barium is rarely found in the living organism.

Thus, the correct answer is option (3).