The statement that best explains how enzymes speed up a chemical reaction is: Enzymes speed up a chemical reaction by lowering the activation energy required for the reactants to come together and react. Enzymes achieve this by binding reactants at their active site, bringing them together in the correct orientation, stabilizing the transition state, and providing an alternative reaction pathway that requires less energy. This reduction in activation energy allows the reaction to occur faster than it would without the enzyme, without being consumed or permanently altered in the process.