Enzymes:

Enzymes speed up the rate of conversion from reactants to products by lowering the activation energy needed for the reaction to occur.

To determine the correct answer, let's break down each option:

a. Increase the amount of energy released in a reaction: This statement is incorrect. Enzymes do not increase the amount of energy released in a reaction. They do not alter the overall energy change of a reaction.

b. Decrease the amount of energy released in a reaction: This statement is also incorrect. Again, enzymes do not affect the total energy change; they simply facilitate the reaction.

c. Catalyze only redox reactions: This statement is incorrect. Enzymes can catalyze a wide range of chemical reactions, not just redox reactions. They can act on diverse substrates and facilitate various types of reactions.

d. Reduce the activation energy needed for a reaction: This statement is correct. Enzymes function by lowering the activation energy required for a chemical reaction to occur. Activation energy is the energy barrier that reactant molecules must overcome in order to reach the transition state and convert into products. By reducing this barrier, enzymes increase the reaction rate without altering the overall energy change.

Therefore, the correct answer is d. Enzymes reduce the activation energy needed for a reaction.