The secondary structure of a protein refers to

The correct answer is Option (2) → The regular folding patterns of the polypeptide chain

The secondary structure is the shape that comes from the regular folding of the polypeptide backbone, as the α-helix or the β-pleated sheet.

Why the other options are incorrect

Option (1) The three-dimensional structure, especially the bonds between amino acid residues that are distant from each other in the polypeptide chain — Incorrect. This describes the tertiary structure, the further folding of the secondary structure that gives fibrous or globular shapes.

Option (3) The spatial arrangement of two or more polypeptide chains (sub-units) with respect to each other — Incorrect. This is the quaternary structure, found only in proteins made of more than one chain.

Option (4) The linear sequence of amino acid residues in the polypeptide chain — Incorrect. The sequence of amino acids is the primary structure.

NCERT (2026-27 edition):
10.2.3 Structure of Proteins – “(ii) Secondary structure of proteins: The secondary structure of protein refers to the shape in which a long polypeptide chain can exist. They are found to exist in two different types of structures viz. α-helix and β-pleated sheet structure. These structures arise due to the regular folding of the backbone of the polypeptide chain due to hydrogen bonding between >C=O and –NH– groups of the peptide bond.”