Final answer:
In an enzyme-catalyzed reaction, the reactants are known as substrates (option C) . These are specific substances that enzymes bind at their active sites to catalyze a chemical reaction.
Step-by-step explanation:
The specific substances that enzymes affect in a biochemical reaction are called the enzyme's substrates. Substrates are the reactants that bind to the active sites of enzymes. The binding of a substrate at an enzyme's active site initiates the chemical reaction, which the enzyme catalyzes. Enzymes are known for their specificity, meaning each enzyme only catalyzes one type of reaction because only substrates with particular shapes and electrical charges can bind to their corresponding enzyme's active site. In the context of the given question, the correct answer would be C. Substrates since they are the reactants in an enzyme-catalyzed chemical reaction.