Answer : (A) Primary structure
Step 1 — Recall the four levels of protein structure
Primary — the exact order (sequence) in which the $\alpha$-amino acids are linked by peptide bonds.
Secondary — local folding of the chain into an $\alpha$-helix or $\beta$-pleated sheet, held by hydrogen bonds.
Tertiary — the overall three-dimensional folding of the whole chain, giving fibrous or globular shapes.
Quaternary — the way two or more separate polypeptide chains (subunits) arrange themselves together.
Step 2 — Match the description
The question asks specifically about the sequence of amino acids, which is exactly the definition of the primary structure.
Step 3 — Note why it matters
The primary structure is held by strong covalent peptide bonds, so it is the only level that survives denaturation. Even a single change in the sequence can alter the protein completely — in sickle-cell anaemia, one amino acid substitution in haemoglobin causes the disease.
