The correct option is C trimeric in nature having 6 antigen binding sites
Immunoglobulin M (IgM) is one of several isotypes of antibodies that are produced by the vertebrates. IgM is the first antibody developed in foetal development. IgM antibodies react to quickly recognise and initiate an immune response by directly neutralising the pathogens or clearing the novel antigens.
IgM is the first antibody to be produced in both primary and secondary immune response. IgM antibodies are the largest antibodies with the biggest size, as it is pentameric (except monomeric on the B cell receptors) in nature having approximately 970 kDa of weight with 10 antigen binding sites and hence they cannot diffuse into tissues.