Final answer:
IgM is the first class of immunoglobulin produced during the classical complement cascade, capable of binding multiple identical antigens due to its pentameric structure. Thus, the option a is the correct answer.
Step-by-step explanation:
The first class of immunoglobulin (Ig) made in the classical complement cascade is IgM. IgM is the primary antibody produced and secreted by B cells during the primary immune response. As a pentamer consisting of five four-chain structures, IgM is able to bind ten identical antigens with its 10 antigen-binding sites. Despite IgG's important role in immune responses, particularly in late primary responses and secondary responses, and its ability to also activate complement proteins, it is IgM that appears first.