Which of the following is an example of a protein?

The general structural formula of $\alpha$-amino acid is $NH_2-CH(R)-COOH$. The group $R$ in tryptophan $(X)$ and histidine $(Y)$ are respectively:

The given structure of $\alpha$-amino acid will exist at which $pH$?
$NH_3^+ - CH(R) - COOH$

Which one of the following statements is not correct?

Which of the following is a basic amino acid?

Which of the following amino acids is an essential amino acid?