Final answer:
A catalyst functions by speeding up a chemical reaction and lowering the activation energy required for the reaction to occur, thereby increasing the rate at which the reactants are converted into products.
Step-by-step explanation:
The function of a catalyst in a reaction is to speed up the reaction (A). It does this not by increasing, but rather by lowering the activation energy (D) required for the reaction to proceed. This is achieved by providing an alternative reaction pathway that requires less energy. As a result, more reactant molecules are able to achieve the transition state more quickly, thus increasing the rate at which products are formed. It's important to note that a catalyst is not consumed in the reaction, but rather, it participates in the reaction to lower the activation energy and is then regenerated for further reaction cycles. Catalysts help a reaction reach equilibrium faster, but they don't change the position of the equilibrium itself.