The correct answer is:
a. They work on a single set of substrates.
Enzymes are typically specific to particular substrates, meaning that each enzyme works on a specific set of substrates to catalyze a particular reaction. The other options are not universally true for all enzymes:
b. They do not change the amount of energy released at the end of a reaction; they only lower the activation energy required for the reaction to proceed.
c. While some enzymes may catalyze more than one reaction, most are specific to a particular type of reaction and substrate.
d. Enzymes are not necessarily broken down immediately following the reaction. They can be reused multiple times to catalyze the same reaction.