Enzymes work by enzymes work by decreasing the potential energy difference between reactant and product adding energy to a reaction reducing ea adding a phosphate group to a reactant increasing the potential energy difference between reactant and product

The appropriate answer is they decrease the potential energy difference between reactant and product. They do this by bringing products and reactants together at the active site on the enzyme molecule. Enzymes are biological catalysts. They increase the rate of a chemical reaction without being used up in the reaction itself. So one molecule of an enzyme is used many times to catalyze the reaction of other molecules. 

The most plausible answer is b, duration. the given amount of time is considered how long it last for.