Answer : (B) Peptide Bond
Step 1 — Recall how a protein is built
A protein is a polymer in which many $\alpha$-amino acid units are linked together. The linkage formed between them is the peptide bond, $-CO-NH-$.
Step 2 — See how it is formed
The $-COOH$ group of one amino acid condenses with the $-NH_2$ group of the next, eliminating a molecule of water :
$$H_2N-CHR-COOH + H_2N-CHR'-COOH \rightarrow H_2N-CHR-\underset{\text{peptide bond}}{CO-NH}-CHR'-COOH + H_2O$$
Step 3 — Rule out the other options
(A) Covalent bond — technically true, since a peptide bond is covalent, but far too general. The question asks for the specific linkage, and every option except (D) describes some kind of covalent bond. "Peptide bond" is the precise answer.
(C) Glycosidic bond — joins monosaccharide units in carbohydrates, not amino acids.
(D) Coordinate bond — found in coordination compounds, where a ligand donates a lone pair to a metal ion.
Step 4 — Note a useful property
The peptide bond has partial double bond character, because the nitrogen lone pair is delocalised onto the carbonyl oxygen. This makes it planar and rigid, which is a major factor in how proteins fold into their secondary and tertiary structures.
Complete hydrolysis of a protein breaks every peptide bond, releasing the individual $\alpha$-amino acids.
