Guanidine group is present in: (A) Histidine, (B) Proline, (C) Tryptophan, (D) Arginine
Answer is D. Amino acid  
Special group








Histidine


Imidazole




Proline


Imino group / Pyrrolidine group




Tryptophan


Indole




ariginine


Guanidine group




Phenylalanine


Benzene ring




Tyrosine


Phenol.