Antibody molecule has:

The correct answer is Option (3) → Four peptide chain

Antibodies are proteins made by B-lymphocytes against a pathogen. An antibody has two light chains and two heavy chains, so it is written as H₂L₂: 2 + 2 = 4 chains in all.

Option (1) Two peptide chain — Incorrect. Two is only the number of light chains, or only the number of heavy chains. The molecule has both pairs.

Option (2) Three peptide chain — Incorrect. The chains come in pairs, so an odd number is not possible.

Option (3) Four peptide chain — Correct. Two light (L) and two heavy (H) chains make the Y-shaped antibody.

Option (4) One polypeptide chain — Incorrect. An antibody is not a single chain; it is built from four chains joined together.

NCERT Extract:
Acquired Immunity – “… The B-lymphocytes produce an army of proteins in response to pathogens into our blood to fight with them. These proteins are called antibodies. The T-cells themselves do not secrete antibodies but help B cells to produce them. Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains. Hence, an antibody is represented as H₂L₂.”