**Core Concept**
The free sulphydryl group, also known as a thiol group (-SH), is a functional group found in certain amino acids. This group is essential for various biochemical reactions, including enzyme catalysis and protein binding. Cysteine is the only standard amino acid with a free sulphydryl group, which distinguishes it from other amino acids.

**Why the Correct Answer is Right**
Cysteine contains a free sulphydryl group due to the presence of a thiol side chain (-SH) in its structure. This unique feature allows cysteine to participate in disulfide bond formation, which is crucial for maintaining the tertiary structure of proteins. The free sulphydryl group in cysteine is also involved in various enzymatic reactions, including the reduction of disulfide bonds and the formation of cystine. The sulphydryl group in cysteine is sensitive to oxidation, which can lead to the formation of cystine.

**Why Each Wrong Option is Incorrect**
**Option B:** Methionine contains a sulfur-containing side chain, but it does not have a free sulphydryl group. Instead, methionine has a thioether group (-S-), which is different from the free sulphydryl group found in cysteine.
**Option C:** Serine is an amino acid with a hydroxyl group (-OH) in its side chain, but it does not have a free sulphydryl group.
**Option D:** Glycine is the simplest amino acid with a single hydrogen atom as its side chain. It does not have a free sulphydryl group.

**Clinical Pearl / High-Yield Fact**
Cysteine is an essential amino acid, meaning it cannot be synthesized by the human body and must be obtained through the diet. A deficiency in cysteine can lead to various health problems, including impaired immune function and increased susceptibility to infections.

**✓ Correct Answer: A. Cysteine**