The structure of protein that is unaffected by heating is:

We need to identify which structure of protein is unaffected by heating. Proteins have four levels of structure: Primary structure is the linear sequence of amino acids held together by covalent peptide bonds; secondary structure refers to local folding patterns (alpha-helix, beta-sheet) held by hydrogen bonds; tertiary structure gives the overall three-dimensional shape maintained by hydrogen bonds, ionic bonds, disulphide bridges, and hydrophobic interactions; and quaternary structure describes the arrangement of multiple polypeptide subunits held by weak intermolecular forces.

Heating causes denaturation, which disrupts the weak bonds (hydrogen bonds, ionic bonds, hydrophobic interactions) that maintain the secondary, tertiary, and quaternary structures. However, the primary structure is maintained because it is held together by strong covalent peptide bonds, which are not broken by moderate heating.

The primary structure of a protein is unaffected by heating. The answer is Option C: primary structure.