iron, oxygen, nickel, copper. sodium, potassium, gold, hydrogen. helium, neon, argon, krypton. carbon, hydrogen, oxygen, nitrogen.
I gave my students this multiple choice question: Plants and animals are composed of organic compounds. Which of the following are the common elements found in organic compounds?

Only one answer is correct among these 4 choices:
- iron, oxygen, nickel, copper.
- sodium, potassium, gold, hydrogen.
- helium, neon, argon, krypton.
- carbon, hydrogen, oxygen, nitrogen.

Could you tell me which one is correct?

carbon, hydrogen, oxygen, nitrogen.