Final answer:
The specificity of an antibody for its antigen is determined by the unique sequences of amino acids in the polypeptide loops of its variable domains.
Step-by-step explanation:
The specificity an antibody has for its antigen is determined by the polypeptide loops in its variable domains. Antibodies are composed of four polypeptides - two heavy chains and two light chains, which together form a Y-shaped structure. The variable domains, found at the ends of these chains, have unique sequences of amino acids that allow the antibody to recognize and bind to specific epitopes on antigens. Therefore, the correct answer to the question is A. polypeptide loops in its variable domains.