Which term describes an enzyme?

A. Substrate B. Reactant C. catalyst D. Product

C. Catalyst.

An enzyme is a biological catalyst that speeds chemical processes.

A substrate is the substance that reacts in contact with an enzyme.

Enzymes are specific when reacting with a substrate. Enzymes react only with certain substrates. It is like:

Enzyme - Substrate

Resembles the relationship of:

Lock - Key

I believe its A, an enzyme is a substrate (or substance) produced by a living thing.