Enzymes increase reaction rates by: (A) Altering the free energy of the action, (B) Inhibiting the backward reaction, (C) Enhancing the forward reaction, (D) Decreasing the energy of activation
Answer is D. MECHANISM OF ENZYME ACTION
Explained by different theories:

Lowering of activation energy
Michaelis-Menten theory
Fischer’s Template theory
Koshland’s Induced Fit theory.

Lowering of Activation Energy
Enzymes lower the activation energy. Activation energy is defined as the energy required to convert all molecules of a reacting substance from ground state to transition state.