Which of the following is a non essential amino acid?

a Histidine
b Methionine
c Tryptophan
d Arginine

Ans: Arginine

Arginine is usually considered non essential because, humans can synthesise it de novo from glutamine, glutamate and proline. But it semi essential because the amount that can be synthesized by the body is inadequate to match the demand. It is required in the diet in young but non essential in adults. 

Hence arginine could be the appropriate answer

Essential amino acids- Those amino acids which cannot be synthesised by the body. Hence they are supplied in the diet

Semi essential amino acids- Growing children require them in food, but not adults.

Non essential amino acids- Amino acids that can be synthesised in the body but not required in the diet