Final answer:
MHC class II molecules are present on antigen-presenting cells, including macrophages, dendritic cells, and B cells. These molecules are crucial for the immune system, helping to activate T cells via presentation of extracellular antigens.
Step-by-step explanation:
MHC class II molecules are found on antigen-presenting cells (APCs). These are known as "professional" antigen-presenting cells, which include macrophages, dendritic cells, and B cells. MHC class II molecules are critical for antigen presentation, which is essential for the initial activation of T cells, particularly CD4+ T helper cells. This is distinct from MHC class I molecules, which are found on all nucleated cells and present antigens to CD8+ cytotoxic T cells.
Also, MHC class II molecules participate in the immune response by presenting extracellular antigens to T cells. For example, macrophages and dendritic cells capture extracellular pathogens, process them, and present fragments of these antigens with MHC class II. Then, T cells recognize these complexes and can thus be activated to respond to the infection. B cells also present antigens to T cells in a somewhat different manner but still involve MHC class II molecules. Hence, the correct answer to which host cells exhibit MHC class II molecules is D. Antigen-presenting cells.