Proteins

An indole group is a fused ring structure composed of a six-membered benzene ring joined to a five-membered nitrogen-containing pyrrole ring. It is a key functional group found in certain biomolecules.
Let us examine each of the given amino acids:
Alanine: Has a side chain —CH₃; it does not contain any ring, aromatic or otherwise.
Tryptophan: Has a side chain —CH₂—indole. The indole structure is a defining feature of tryptophan.
Glycine: Has the simplest side chain —H; no ring present.
Proline: Contains a five-membered ring, but it is a pyrrolidine ring (not an indole group).
Therefore, among the options, only Tryptophan contains an indole group in its side chain.