An imino acid is

Options

A. Leucine
B. Phenylalanine
C. Lysine
D. Proline.

Answer

### Step-by-Step Solution:

1. **Understanding Amino Acids**:
- Amino acids are organic compounds that serve as the building blocks of proteins. They typically have a basic structure that includes an amino group (-NH2), a carboxyl group (-COOH), and a variable side chain (R group) that distinguishes one amino acid from another.

2. **Defining Imino Acids**:
- Imino acids are a specific type of amino acid where the amino group is replaced by an imino group (-NH). This structural difference is crucial in identifying imino acids.

3. **Analyzing the Given Options**:
- The question provides four amino acids: phenylalanine, lysine, proline, and asks which one is an imino acid.
- **Phenylalanine**: Contains a standard amino group (-NH2) and is not an imino acid.
- **Lysine**: Also contains a standard amino group (-NH2) and is not an imino acid.
- **Proline**: Contains an imino group (-NH) instead of a standard amino group. This makes proline an imino acid.

4. **Conclusion**:
- Among the options provided, proline is the only imino acid due to the presence of the imino group in its structure.

### Final Answer:
- An imino acid is **proline**.