## Core Concept
Carboxypeptidase is a pancreatic exopeptidase enzyme that hydrolyzes peptides and proteins at the carboxy-terminal (C-terminal) end. It plays a crucial role in protein digestion. The enzyme requires a specific metal ion as a cofactor to function properly.

## Why the Correct Answer is Right
The correct answer, **Zinc (Zn)**, is a crucial cofactor for carboxypeptidase. Zinc is essential for the catalytic activity of carboxypeptidase, facilitating the hydrolysis of peptide bonds. The zinc ion helps to stabilize the transition state during the enzymatic reaction, enabling efficient peptide bond cleavage.

## Why Each Wrong Option is Incorrect
* **Option A: Iron (Fe)** - Iron is not directly involved as a cofactor in the enzymatic activity of carboxypeptidase. While iron is crucial for various other biological processes, such as oxygen transport and enzyme functions (e.g., cytochromes), it is not associated with carboxypeptidase activity.
* **Option B: Calcium (Ca)** - Calcium ions play significant roles in many biological processes, including muscle contraction, neurotransmitter release, and blood coagulation. However, calcium is not directly involved as a cofactor for carboxypeptidase.
* **Option D: Potassium (K)** - Potassium is important for maintaining membrane potential, nerve function, and muscle contraction. It does not serve as a cofactor for carboxypeptidase.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **zinc deficiency can impair enzyme function and immune response**. Zinc is a cofactor for numerous enzymes, including carboxypeptidase, and its deficiency can lead to growth retardation, impaired immune function, and increased susceptibility to infections.

## Correct Answer Line
**Correct Answer: C. Zinc.**