Final answer:
The correct option is D). Cytotoxic T cells mainly express the CD8 co-receptor and target MHC Class I molecules, which present antigens from intracellular pathogens for immune response.
Step-by-step explanation:
The vast majority of cytotoxic T cells express the CD8 co-receptor and are restricted to MHC Class I molecules. Therefore, the correct option is b) CD8 / Class I. These cells are essential for the immune response by directly killing infected host cells. The MHC Class I molecules present antigens from intracellular pathogens like viruses, signaling cytotoxic T cells to eliminate the infected cells and prevent the further spread of the infection.