Final answer:
d) IgA forms dimers, particularly in exocrine gland secretions for mucosal immunity, while IgM forms pentamers, which are the first antibodies made during primary immune responses.
Step-by-step explanation:
The immunoglobulin that forms dimers is IgA, whereas the one that forms pentamers is IgM. IgA can exist as a monomer in the blood and as a dimer in exocrine gland secretions. These dimers are important for mucosal immunity. On the other hand, IgM is a monomer when it functions as a receptor on the surface of B cells, but it forms a pentamer when secreted in response to specific pathogens. This pentameric form of IgM is the first antibody made by plasma cells during primary immune responses and is capable of binding to ten identical antigens due to its structure.