the total mass of available carbon the way organisms use carbon the physical state of carbon-containing molecules the molecule in which a particular carbon atom is found
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: Which of the following remains constant in Earth's carbon cycle?
Possibilities:
- the total mass of available carbon
- the way organisms use carbon
- the physical state of carbon-containing molecules
- the molecule in which a particular carbon atom is found

the total mass of available carbon