You have isolated a previously unstudied protein, identified its complete structure in detail, and determined that it is an enzyme that catalyzes the breakdown of a large substrate. you notice it has two binding sites: one large and one small. speculate about how this enzyme might function.

This enzyme has two binding sites: one for its substrate and another for the union of its cofactor or coenzyme. The enzyme needs the interaction with its coenzyme ro catalyze the breakdown of its substrate.  

Explanation:

Enzymes are biological catalysts, it means molecules that facilitates a reaction from a reactant to its product. The reactant that interacts with the enzyme is called its substrate.

         Enzyme

Raectant ⇒ Product

Different enzymes have different degrees of specificity, there are enzymes that only catalyze the reaction for a particular compound and others that catalyze the reaction of a family of compounds.

Some enzymes requiere the interaction with a cofactor (a metalic ion or an organic molecule) known as coenzyme. This is the case of the isolated protein that catalyze the breakdown of a large substrate: it has a large binding site for its substrate and a small one for the union of its cofactor. Only when the enzimes binds the substrate and the coenzyme, the reaction take place.

the answer to your question is:

c. complementary base-pairing