## Core Concept
The question tests understanding of amino acid structure, specifically the presence of an imino group. Amino acids are organic compounds containing both a carboxyl (-COOH) and an amino (-NH2) group. An imino group, however, refers to a functional group with a carbon-nitrogen double bond (C=N).

## Why the Correct Answer is Right
Proline is unique among the standard amino acids because it has an imino group. This is due to its side chain, which forms a cyclic structure by linking back to the alpha-amino group, effectively making the amino group an imino group. This cyclic structure is crucial for proline's role in protein structure, particularly in turns and loops, due to its conformational rigidity.

## Why Each Wrong Option is Incorrect
* **Option A:** Glycine is the simplest amino acid with a side chain that consists of just a hydrogen atom. It does not contain an imino group.
* **Option B:** Alanine has a methyl group as its side chain and does not have an imino group.
* **Option D:** Leucine has a large, hydrophobic side chain and lacks an imino group.

## Clinical Pearl / High-Yield Fact
Proline's unique structure makes it critical in the formation of collagen, a major structural protein found in connective tissue. The imino group of proline contributes to the stability of collagen's triple helix structure. A memorable point is that hydroxyproline, a derivative of proline, is crucial for collagen stability, and its synthesis requires vitamin C.

## Correct Answer: C. Proline.