The amino acid that contains an indole ring in its side chain is:
Tryptophan has an indole ring (a benzene ring fused to a pyrrole ring) in its side chain, making it aromatic and hydrophobic. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.
Ref: NCERT Chemistry Textbook for Class XI and XII, Chapter: Relevant Chemistry topic covering principles and examples as per NCERT.