The correct answer is:
a. They work on a single set of substrates.
Enzymes are specific to their substrates, meaning that each enzyme typically catalyzes a reaction involving a specific substrate or a group of closely related substrates. While there are some exceptions where enzymes can act on similar substrates, the general rule is that enzymes exhibit specificity for their substrates.
The other options are not true for all enzymes:
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b. They break down immediately following the reaction it catalyzes. This is not true; enzymes are not consumed in the reaction and can be reused.
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c. They catalyze a variety of different reactions. This is misleading because most enzymes are specific to a particular reaction.
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d. They change the amount of energy released at the end of a reaction. Enzymes lower the activation energy of a reaction but do not change the overall energy released or absorbed in the reaction.