Question

enzymes work by ___.
reducing ( e _ { a } )
adding a phosphate group to a reactant
decreasing the potential energy difference between reactant and product
adding energy to a reaction
increasing the potential energy difference between reactant and product

Explanation:

Enzymes are biological catalysts. Catalysts work by reducing the activation energy ($E_A$) of a reaction. Activation energy is the minimum energy required for a reaction to occur. By lowering $E_A$, enzymes speed up the reaction. Adding a phosphate group (phosphorylation) is a specific reaction type (like in some metabolic pathways) but not how enzymes generally work. Enzymes do not change the potential energy difference between reactant and product (that's determined by thermodynamics of the reaction itself). They also do not add energy to the reaction; instead, they provide an alternative reaction pathway with lower $E_A$.

Answer:

reducing $E_A$