Protein structure is organised into hierarchical levels, and denaturation selectively targets the higher levels while sparing the covalent backbone sequence. The primary structure is the covalent sequence of amino acids held by peptide bonds; the secondary structure (alpha-helix and beta-pleated sheet) and tertiary structure (the overall three-dimensional fold) are stabilised mainly by weaker interactions such as hydrogen bonds, hydrophobic contacts, and disulfide bridges. Heat, strong acid, or other denaturing agents disrupt these weaker stabilising forces, so the protein unfolds and loses its native shape and biological activity, often becoming insoluble and coagulating, as seen when an egg sets. Crucially, the peptide bonds of the primary sequence are not broken, so the amino acid order is preserved; this rules out the option claiming hydrolysis of every peptide bond. The selective cysteine peptide-bond option is meaningless, and proteins do not contain N-glycosidic linkages, which belong to nucleic acids. This is the NCERT definition of denaturation. As a sanity check, denatured proteins can sometimes refold if conditions are gentle, which is only possible because the unbroken primary sequence still encodes the native structure.