The substance on which the enzyme acts is called the
enzyme
substrate
catalyst
buffer

The best answer is substrate.

The molecule to which an enzyme binds is called a substrate. Substrates get specifically selected by enzymes because enzymes will only bond with the correct substrate that they have been genetically designed to bond with. 

The substrate binds to a small area on the enzyme referred to as the  active site and they form an enzyme - substrate complex.

Once the reaction between substrate and enzyme is complete, the substrate will have become a product.