The correct statement about ALL enzymes is:
c) They work on a single set of substrates.
Enzymes are typically specific to particular substrates due to their active site structure, which fits the substrate(s) they act upon, like a key fits a lock. The other options are not true for all enzymes:
a) While many enzymes do catalyze specific types of reactions, they do not all catalyze a variety of different reactions; most have a specific function.
b) Enzymes lower the activation energy of a reaction but do not change the overall energy released or consumed in a reaction.
d) Enzymes are not broken down immediately following the reaction; they can be used repeatedly for many reactions before they are degraded.