Final answer:
MHC molecules are critical for T-cell activation, where MHC II molecules on antigen-presenting cells display processed antigens to helper T cells, which is the correct option, C.
Step-by-step explanation:
The correct answer to the student's question is C. T-cell activation. Major Histocompatibility Complex (MHC) molecules are crucial for the activation of T cells. When a pathogen is phagocytized and digested by cells such as macrophages or dendritic cells, a fragment of the pathogen, known as an antigen, is presented on the cell's surface by MHC molecules. There are two classes of MHC molecules: MHC I and MHC II. MHC I molecules are found on all nucleated cells and present antigens that are derived from inside the cell, including those from intracellular pathogens, marking infected cells for destruction by cytotoxic T cells. MHC II molecules, on the other hand, are expressed only on the surface of professional antigen-presenting cells (APCs) such as macrophages, B cells, and dendritic cells, and they present processed foreign antigens from these cells to helper T cells. This presentation is vital for the activation of T cells within the adaptive immune system, allowing them to mount a proper immune response.