The answer to the question is A active site. Let's go through each choice in turn. The active site of the enzyme is the exact place on the enzyme that the substrate goes into and where the actual reaction occurs. So this is the most important part of the enzyme since this is where it performs its actual chemical function. Activation energy is not a part of an enzyme molecule. It is the energy barrier that the enzyme helps to overcome to make the overall chemical reaction occur. The primary structure is the overall sequence of amino acids that make up the enzyme. It is certainly important to the nature of the enzyme but it is not the single most important part for its function. Finally, the transition state is not a part of the enzyme. It is an intermediate state that the substrate passes through on its way to becoming the product.