Which of the following statements best describes enzymes?

Enzymes are biological catalysts that play a crucial role in facilitating chemical reactions within living organisms. The key characteristic that makes option B the best description of enzymes is their ability to lower the activation energy required for a reaction to take place. By doing so, enzymes increase the rate of the reaction, allowing it to occur more efficiently and under milder conditions than would otherwise be possible.

When enzymes bind to substrates, they form an enzyme-substrate complex, which stabilizes the transition state and reduces the energy barrier for the reaction. This is essential for biological processes, as many reactions would proceed too slowly to sustain life without the aid of enzymes.

The other statements do not accurately describe the nature or function of enzymes. Enzymes are not consumed during reactions; they can be used multiple times, remaining unchanged after the reaction is complete. Additionally, enzymes do not act as substrates; rather, they interact with substrates to catalyze reactions. Finally, enzymes require the presence of substrates to function; without substrates, they cannot perform their catalytic roles.