An enzyme-substrate complex is composed of which of the following?

Eliminate incorrect options: An enzyme and its product is not correct because the product is formed after the enzyme-substrate complex dissociates. Similarly, a substrate and an inhibitor is incorrect because inhibitors typically prevent substrate binding or enzyme activity. A substrate and a cofactor is also incorrect because cofactors assist enzyme function but are not part of the enzyme-substrate complex.

Focus on the correct option: The enzyme-substrate complex specifically refers to the enzyme bound to its substrate, which is the intermediate state before the substrate is converted into the product.