Which amino acid has an aromatic ring?

Within the list shown below, the correct pair of structures of alanine in $pH$ ranges $2-4$ and $9-11$ is:
$I$. $H_{3}N^{+}CH(CH_{3})COOH$
$II$. $H_{2}NCH(CH_{3})COO^{-}$
$III$. $H_{3}N^{+}CH(CH_{3})COO^{-}$
$IV$. $H_{2}NCH(CH_{3})COOH$

The main structural feature of proteins is

The optically inactive amino acid is

Identify the essential amino acids from below $:$
$(A)$ Valine
$(B)$ Proline
$(C)$ Lysine
$(D)$ Threonine
$(E)$ Tyrosine
Choose the correct answer from the options given below $:$

The structure of protein that is unaffected by heating is ... .