Final answer:
The antibody that binds complement most effectively is IgM, which is structured as a pentamer. IgG can also activate complement proteins, but not as efficiently as IgM. Therefore, the correct option is D.
Step-by-step explanation:
The question asks which class of antibodies is capable of binding complement proteins. The correct answer is D. IgM. IgM is uniquely structured as a pentamer that allows it to effectively bind complement due to its multiple antigen-binding sites. Furthermore, it is usually the first antibody produced in a primary response. On the other hand, IgG, although not as effective as IgM, can also activate complement proteins. This function is part of the immune system's ability to fight off pathogens, with IgM's pentameric arrangement making it particularly adept at complement activation.