Check option A

Statement: All naturally occurring amino acids except glycine contain one chiral centre.

• Glycine is achiral because its side chain is $H$.
• However, the statement says all other naturally occurring amino acids have one chiral centre.
• This is false because some amino acids have more than one chiral centre. For example, isoleucine and threonine each contain two chiral centres.

Therefore, A is incorrect.

Check option B

Statement: Amino acid, cysteine can easily undergo dimerisation due to the presence of free SH group.

• Cysteine contains a thiol group, $-SH$.
• Two molecules of cysteine can be oxidized to form a disulfide linkage, $-S-S-$, producing cystine.
• This is a common dimerisation reaction: $2\,\text{Cysteine} \rightarrow \text{Cystine}$

Therefore, B is correct.

Check option C

Statement: Glutamic acid is the only amino acid that contains a $-COOH$ group at the side chain.

• This is false because aspartic acid also contains a side-chain carboxylic acid group.
• Both aspartic acid and glutamic acid are acidic amino acids.

Therefore, C is incorrect.

Check option D

Statement: All naturally occurring amino acids are optically active.

• Glycine is naturally occurring and optically inactive because it has no chiral centre.

Therefore, D is incorrect.

Conclusion

The only correct statement is: $\boxed{\text{B}}$

Comparison with stored correct answer

• Derived answer: B
• Stored correct answer: B

They match.