Which structure of protein remains intact after coagulation of egg white on boiling?

$A$ 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

Out of the following,which type of interaction is responsible for the stabilisation of $\alpha$-helix structure of proteins?

The value of $x$ in compound '$D$' is $.........$.

Identify the amino acids obtained by hydrolysis of the following compound: $NH_2-CH(CH_3)-CO-NH-CH_2-COOH$

Which of the following amino acids is not optically active?