An amino acid is

Options

A. Renin
B. Pepsin
C. Cystine
D. Proline.

Answer

To determine which of the given options is an amino acid, we need to understand the definition and structure of amino acids.

### Step-by-Step Solution:

1. **Definition of an Amino Acid**:
An amino acid is an organic molecule that contains a primary amino group (-NH2), a carboxyl group (-COOH), and a unique side chain (R group) that varies among different amino acids.

2. **Analyzing the Options**:
- **Renin**: This is an enzyme involved in blood pressure regulation. It is a protein, not an amino acid. Therefore, this option is incorrect.
- **Pepsin**: This is also an enzyme that helps in digestion. Like renin, it is a protein and not an amino acid. Thus, this option is also incorrect.
- **Cysteine**: Cysteine is indeed an amino acid. It contains a sulfur atom in its side chain and is important for protein structure. This option is correct.
- **Proline**: Proline is classified as an amino acid, but it is unique because it has a secondary amine group instead of a primary amine. While it is an amino acid, the question specifically asks for a standard amino acid, and proline's structure is somewhat different from typical amino acids.

3. **Conclusion**:
Based on the definitions and analysis of the options, the correct answer is **Cysteine**, as it is a standard amino acid.

### Final Answer:
The correct answer is **Cysteine**.