Question

what is a substrate?
a molecule that acts as a catalyst
a place on the enzyme where the reaction occurs
a place on the product where the enzyme fits
a reactant that is catalyzed by an enzyme

Explanation:

In biochemistry, a substrate is the reactant molecule upon which an enzyme acts to catalyze a chemical reaction. A molecule that acts as a catalyst is an enzyme. The place on the enzyme where the reaction occurs is the active - site. There is no such thing as a place on the product where the enzyme fits in the context of substrate - enzyme interactions.

Answer:

a reactant that is catalyzed by an enzyme