Answer : (A) primary structure of proteins

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 into an $\alpha$-helix or $\beta$-pleated sheet, held by hydrogen bonds

Tertiary — the overall three-dimensional folding of the whole chain

Quaternary — the arrangement of two or more separate polypeptide chains (subunits)

Step 2 — Match the description

The question speaks specifically of the sequence of amino acids, which is precisely the definition of the primary structure.

Step 3 — Note what holds it together

The primary structure is held by strong covalent peptide bonds $(-CO-NH-)$. This is why it is the only level that survives denaturation — heating or a pH change destroys the secondary and tertiary structures but leaves the sequence intact.

Step 4 — See why the sequence matters so much

Even a single change in the sequence can alter the protein completely.

In sickle-cell anaemia, one amino acid substitution in the haemoglobin chain — glutamic acid replaced by valine — is enough to distort the red blood cells and cause the disease.

The primary structure ultimately determines all the higher levels, since the sequence dictates how the chain folds.