Proteins are condensation polymers of alpha-amino acids, each of which carries an amino group and a carboxyl group attached to the same alpha carbon. When two such molecules join, the carboxyl group of one residue reacts with the amino group of the next, eliminating a molecule of water and creating an amide linkage, which in protein chemistry is specifically called a peptide bond. This -CO-NH- bond has partial double-bond character due to resonance, giving the planar, rigid backbone that underlies secondary structure. The option proposing condensation of two amino groups with loss of ammonia is wrong because no such linkage defines peptides. Two carboxyl groups losing carbon dioxide would describe a different decarboxylative process, not peptide formation. Side-chain hydroxyls may form other interactions but do not constitute the defining backbone bond. This is exactly the NCERT definition of the peptide bond in the Biomolecules chapter. As a sanity check, hydrolysis of a peptide bond regenerates the free carboxyl and amino groups, confirming that those two groups are the ones that condensed in the first place.