Why do we say that "an enzyme is reusable"?
a) the enzyme does not actively take part in the reaction. it just indicates that the reaction is complete.
b) enzymes react with the substrate molecules to form products and become denatured in the process.
c) enzymes ensure that the reaction continues without inhibition even if conditions such as temperature and ph are changed.
d) the products of the reaction are released from the active sites of the enzyme, allowing other substrate molecules to bind with the sites.

Option D, The products of the reaction are released from the active sites of the enzyme, allowing other substrate molecules to bind with the sites.

Explanation:

Enzymes are either chemical/biochemical/biological substances that do not take part in a reaction but assist in carrying out a chemical reaction.

Usually all enzymes have an active site on which the substrate  attaches by forming some minor surface bonds and once the chemical reaction is complete the product formed at the active site is released thereby making it available for new attachments.

Hence, option D is correct.

gold i'm pretty sure i might be wrong

i believe your answer would be c.. : )