What is a substrate?
a molecule that changes state
a place where the reaction occurs
a place on the product where the enzyme fits
a reactant that is catalyzed by an enzyme

Option (d) is the correct answer.

Explanation:

Enzymes which bind with chemical reactants are known as substrate. When a substrate combines with an enzyme it results in the formation of an enzyme-substrate complex.

The reaction will be as follows.

Further, this enzyme-substrate complex upon decomposition gives product. Enzyme is a catalyst which increases the rate of reaction without itself getting consumed in the reaction. Therefore, the enzyme will remain as such.

Thus, we can conclude that substrate is a reactant that is catalyzed by an enzyme.