Q.A disulfide link gives rise to the following structure of protein.
a. Primary
b. Secondary
c. Tertiary
d. Quaternary

Step 1. Section 14.3.4c lists the forces that stabilise a protein's tertiary structure: hydrogen bonding, dipole-dipole attraction, electrostatic attraction, London dispersion forces, and -- specifically -- disulfide bonds formed by oxidative coupling of nearby -SH groups on cysteine residues.

Step 2. Rule out the other levels: primary structure is just the amino-acid sequence (peptide bonds only, no disulfide involvement); secondary structure (alpha-helix/beta-sheet) arises from backbone N-H...O=C hydrogen bonding, not disulfide bonds; quaternary structure describes multiple SEPARATE chains associating (disulfide bonds CAN sometimes link separa …

Recall which structural level's stabilising-forces list explicitly includes disulfide bonds …

• Confusing disulfide bonds with the hydrogen-bonding pattern that specifically defines secondary structure (alpha-helix/beta-sheet). …