Which of the following acids behaves like a salt in aqueous solution?

The structure of $Gly-Ala$ is

Which amino acid has an aromatic ring?

$A$ protein undergoes reversible thermal denaturation from its initial state $N$ to denatured state $D$ according to $N \rightleftharpoons D$. At $60 ^\circ C$, the concentrations of both $N$ and $D$ are equal at equilibrium, and the standard enthalpy change of denaturation is $666 \text{ kJ mol}^{-1}$. The standard entropy change ($\Delta S^\circ$ in $\text{kJ K}^{-1} \text{mol}^{-1}$) of the protein upon denaturation at $60 ^\circ C$ is closest to

What is the total number of essential amino acids?

There are $20$ naturally occurring amino acids. The maximum number of tripeptides that can be obtained is