Final answer:
Lactose is referred to as a substrate because it is the molecule on which the enzyme acts.
Step-by-step explanation:
The correct answer is a) Substrate.
Lactose is referred to as a substrate because it is the molecule on which the enzyme acts. In this case, the enzyme lactase breaks down lactose into its two monomers, galactose and glucose. The lactose molecule is the substrate for the lactase enzyme.