Which of the following amino acids is optically inactive?

Globular protein is present in -

$A$ dipeptide,$X$ on complete hydrolysis gives $Y$ and $Z$. $Y$ on treatment with aqueous $HNO_2$ produces lactic acid. On the other hand,$Z$ on heating gives the following cyclic molecule.
Based on the information given,the dipeptide $X$ is:

If there are $20$ naturally occurring amino acids,what is the maximum number of tripeptides that can be formed?

Find the isoelectric point $(pI)$ of the given amino acid: $HOOC-CH_2-CH_2-CH(NH_3^+)-COOH$. Given: $pK_{a1} = 2$,$pK_{a2} = 4$,$pK_{a3} = 9$.

Which of the following is a molecular disease?