Final answer:
An antibody consists of four polypeptide chains, specifically two heavy chains and two light chains, arranged in a 'Y' shape, with the antigen-binding site involving both types of chains at the tips of the 'Y'.
Step-by-step explanation:
An antibody molecule is made up of two heavy chains and two light chains. This four-chain structure is often depicted in a 'Y' shape, where the two heavy chains form the stem and the two light chains pair with the branches of the heavy chains. The antigen-binding site is formed at the ends of the 'Y', involving both the light and heavy chains. Each chain has variable domains that are responsible for binding with the antigen, and constant domains that define the class of the antibody and its role in the immune response.
The correct answer to the question is: b) Two heavy chains; two light chains. Antibodies, also known as immunoglobulins, consist of these two types of polypeptide chains, which are held together by disulfide bonds. Notably, the chains contain different regions with specific functions; for example, light chains have one variable and one constant domain, while heavy chains contain one variable domain and either three or four constant domains, depending on the class of antibody.