Final answer:
The correct option is B. MHC II molecules have a floor that consists of the CD4 protein. It helps facilitate the interaction between the MHC II molecule and T cells.
Step-by-step explanation:
MHC II molecules have a floor that consists of the CD4 protein. The CD4 protein is a receptor that binds to MHC II molecules on antigen-presenting cells, such as macrophages, dendritic cells, and B cells.
The CD4 protein helps facilitate the interaction between the MHC II molecule and T cells.