Identify the correct statement among the following:

We test each statement one by one, recalling the standard structures and properties of the 20 common (protein-forming) amino acids.

Case A: “All naturally occurring amino acids except glycine contain one chiral centre.”
Glycine has the structure $$NH_2\!-\!CH_2\!-\!COOH$$; since the $$\alpha$$-carbon bears two identical $$H$$ atoms, it is achiral.
However, amino acids such as isoleucine and threonine possess two chiral centres each (one at the $$\alpha$$-carbon and another in the side chain). Thus the phrase “one chiral centre” is wrong. Hence statement A is incorrect.

Case B: “All naturally occurring amino acids are optically active.”
Because glycine is achiral, it is not optically active. Therefore this statement is also incorrect.

Case C: “Glutamic acid is the only amino acid that contains a -COOH group at the side chain.”
Glutamic acid has the side chain $$-CH_2-CH_2-COOH$$, but aspartic acid also contains an extra carboxyl group in its side chain $$-CH_2-COOH$$. Thus glutamic acid is not unique in this respect, so statement C is incorrect.

Case D: “Amino acid, cysteine easily undergo dimerization due to the presence of free SH group.”
Cysteine contains the side chain $$-CH_2-SH$$. Two cysteine molecules readily oxidise to form a disulfide linkage:
$$2 \,HS-CH_2-CH(NH_2)COOH \;\longrightarrow\; HOOCCH(NH_2)-CH_2-S-S-CH_2-CH(NH_2)COOH + 2H^+ + 2e^-$$
The dimer formed is called cystine. Hence statement D is correct.

Therefore the correct choice is Option D (Option 4).