Answer : (B) Primary structure
Step 1 — Recall the four levels of protein structure
Primary structure — the exact order (sequence) in which amino acids are linked by peptide bonds in the polypeptide chain.
Secondary structure — the local folding of the chain into shapes such as the $\alpha$-helix or $\beta$-pleated sheet, held by hydrogen bonds.
Tertiary structure — the overall three-dimensional folding of the whole chain, giving fibrous or globular proteins.
Quaternary structure — 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 the definition of the primary structure.
Step 3 — Note its importance
Even a single change in this sequence can completely alter the protein's function. Sickle-cell anaemia, for instance, is caused by one amino acid substitution in haemoglobin.
