Final answer:
The correct answer is 'b. catalyst.' A catalyst accelerates a chemical reaction without being consumed, and can be recovered and reused. It lowers the reaction's activation energy and increases the rate of collisions among reactants.
Step-by-step explanation:
A catalyst is something that speeds up a reaction without actually getting used up in the process. The correct answer is 'b. catalyst.' In chemistry, a catalyst functions as a chemical change agent, allowing the reaction to proceed at a faster rate by providing an alternative pathway with a lower activation energy. Catalysts participate in reactions and increase the rate at which atoms, ions, and molecules collide, without altering the equilibrium of the reaction. After the reaction, catalysts can be recovered unchanged and reused.