Biology : PU-CET (U.G) 2015 Panjab University Entrance Exam With Answers

The correct answer is D: Alanine.

Alanine is a non-essential amino acid, meaning that it can be synthesized by the human body and does not need to be obtained from the diet. Essential amino acids, on the other hand, cannot be synthesized by the body and must be obtained from the diet.

Leucine (A), Lysine (B), and Methionine (C) are all essential amino acids that must be obtained from the diet. Leucine is involved in muscle protein synthesis, Lysine is important for collagen synthesis and immune function, and Methionine is important for methylation reactions and the synthesis of cysteine and taurine.