Standard single-letter IUPAC symbols for amino acids:

• Lysine: Code is K $\rightarrow$ (III)
• Tryptophan: Code is W $\rightarrow$ (I)
• Tyrosine: Code is Y $\rightarrow$ (IV)
• Glutamine: Code is Q $\rightarrow$ (II)
  Correct match: (A)-(III), (B)-(I), (C)-(IV), (D)-(II).

By convention, peptides are written starting from the N-terminal amino acid on the left to the C-terminal amino acid on the right.
In Gly-Ala (glycylalanine):

• Glycine ($\text{H}_2\text{N}-\text{CH}_2-\text{COOH}$) provides the free $-\text{NH}_2$ group and participates via its $-\text{COOH}$ group.
• Alanine ($\text{H}_2\text{N}-\text{CH}(\text{CH}_3)-\text{COOH}$) participates via its $-\text{NH}_2$ group and provides the free $-\text{COOH}$ group.

Combining them yields $\text{H}_2\text{N}-\text{CH}_2-\text{CO}-\text{NH}-\text{CH}(\text{CH}_3)-\text{COOH}$.

Levels of protein structure:

• Linear sequence of amino acids linked by peptide bonds represents the Primary structure $\rightarrow$ (I)
• Regular local conformations such as $\alpha$-helix and $\beta$-pleated sheet stabilised by hydrogen bonding represent Secondary structure $\rightarrow$ (II)
• Overall three-dimensional folding of polypeptide chain, classifying proteins into fibrous and globular shapes, represents Tertiary structure $\rightarrow$ (IV)
• Spatial arrangement of two or more polypeptide subunits with respect to each other represents Quaternary structure $\rightarrow$ (III)

Correct match: (A)-(I), (B)-(II), (C)-(IV), (D)-(III).