To solve this problem, we need to identify the cofactor of the enzyme carboxypeptidase.$\newline$Carboxypeptidase is a type of enzyme that is involved in protein digestion. It specifically cleaves the$\newline$terminal amino acid from the carboxyl end of a protein or peptide.$\newline$Let's analyze the options given:$\newline \bullet$Option 1: Flavin - Flavin is a cofactor associated with enzymes like flavoproteins, which are involved in$\newline$redox reactions. It is not the cofactor for carboxypeptidase.$\newline \bullet$Option 2: Haem - Haem is a cofactor found in hemoproteins like hemoglobin and cytochromes,$\newline$which are involved in oxygen transport and electron transfer, respectively. It is not the cofactor for carboxypeptidase.$\newline \bullet$Option 3: Zinc - Zinc is a metal ion that acts as a cofactor for many enzymes, including carboxypeptidase.$\newline$It plays a crucial role in the catalytic activity of the enzyme by stabilizing the transition state and$\newline$activating water molecules for nucleophilic attack on the peptide bond.$\newline \bullet$Option 4: Niacin - Niacin is a vitamin that is a precursor to NAD and NADP, which are cofactors in redox reactions.$\newline$It is not the cofactor for carboxypeptidase.$\newline$Based on the analysis, the correct cofactor for carboxypeptidase is Zinc.$\newline$Therefore, the correct option is Option 3: Zinc.