Most stable amino acid at physiological pH is: (A) Histidine, (B) Lysine, (C) Arginine, (D) Glycine
Answer is A. Histidine plays unique roles in enzymatic catalysis. The pKa of its imidazole proton permits histidine to function at neutral pH as either a base or an acid catalyst  without  the  need  for  any  environmentally  induced shift. 
Harper's Illustrated Biochemistry 30 th edition page no 22