Which of the following functions to speed up a chemical reaction by reducing the required activation energy?

The correct answer is that a catalyst speeds up a chemical reaction by reducing the required activation energy. Catalysts are substances that increase the rate of a reaction without themselves undergoing any permanent chemical change. They achieve this by providing an alternative pathway for the reaction with lower activation energy, allowing more reactant molecules to successfully collide and react at a given temperature.

While enzymes are specialized types of catalysts that are biological in nature, the term "catalyst" encompasses both biological and non-biological substances that facilitate chemical reactions. Therefore, while enzymes can also speed up reactions, the broader term for any substance that does so is catalyst, making it the most precise answer in this context.

Substrates refer to the reactants that enzymes act upon and do not play a role in altering activation energy themselves. Inhibitors, on the other hand, are substances that decrease the rate of a reaction or prevent it altogether, which is the opposite of what the question is asking. Thus, the concept of a catalyst encompasses the essential function required, making it the correct choice.