Which of the following is an organic compound?

A. ammonia (NH3)

B. calcium sulfide (CaS)

C. octane (C8H18)

D. sulfur trioxide (SO3)

I think the correct answer from the choices listed above is option C. It would be octane that is an organic compound. They are, any of a large class of chemical compounds in which one or more atoms of carbon are covalently linked to atoms of other elements, most commonly hydrogen, oxygen, or nitrogen.

Most of organic compounds consists of C and H atoms. Hence C. octane (C8H18) is the correct option.