which letter represents a reaction with an enzyme present?

Brief Explanations:

Enzymes are biological catalysts that lower the activation - energy of a reaction. In a reaction energy diagram, the reaction with an enzyme has a lower peak (activation energy) compared to the reaction without an enzyme. The dashed line represents the reaction with the enzyme present as it has a lower activation energy. Here, the curve with the lower peak (activation energy) is the one represented by the dashed line, which corresponds to the letter C.

Answer:

C