Zinc, an essential trace element for most organisms, is present in the active site of the enzyme carboxypeptidase. the zinc most likely functions as

The correct answer is "a cofactor necessary for enzyme activity". 

Cofactors are non-organic molecules that are required for the enzyme to work. These are usually minerals such as zinc, copper, selenium, etc. Coenzymes on the other hand are organic molecules, usually proteins, that are required for the enzyme to work.