Identify the essential amino acid from the following.

Assertion : Carboxypeptidase is an exopeptidase.
Reason : It cleaves the $N$-terminal bond.

The number of possible tripeptides formed involving alanine $(ala)$, glycine $(gly)$, and valine $(val)$, where no amino acid has been used more than once is:

Assertion $(A)$: In aqueous solution,amino acids exist in dipolar ion form.
Reason $(R)$: Most of the naturally occurring amino acids have $L$-configuration.
The correct option among the following is:

The correct structure of the dipeptide gly-ala is

Identify the acidic amino acid from the following (represented by using three-letter symbols).