For its activity, carboxypeptidase requires

Many enzymes need a non-protein cofactor for their catalytic activity; when the cofactor is a metal ion it forms coordination bonds with the active site and the substrate.
Carboxypeptidase is a proteolytic enzyme that removes amino acids from the carboxyl end of a peptide.
NCERT gives zinc as the cofactor of carboxypeptidase.
Copper is associated with enzymes such as cytochrome oxidase, iron with haem of catalase and peroxidase, and niacin is a vitamin forming the coenzymes NAD and NADP.
Hence carboxypeptidase requires zinc.