Final answer:
MHC-II loading with antigen fragments occurs during the fusion of vesicles containing MHC-II proteins and vesicles with digested pathogens, following phagocytosis by APCs, facilitating antigen presentation on the cell surface. Therefore, the correct option is b.
Step-by-step explanation:
MHC-II loading (attaching to the antigen) occurs during the fusion of vesicles containing MHC-II proteins and vesicles containing digested pathogens. After phagocytosis by antigen-presenting cells (APCs), the phagocytic vesicle fuses with an intracellular lysosome to form a phagolysosome. Within this phagolysosome, pathogenic components are broken down into fragments; these fragments are then loaded onto MHC class II molecules. The MHC proteins, now carrying the antigens, are released from the vesicle and transported to the outer surface of the cell membrane for antigen presentation to T cells, such as Helper T-cells, which are essential for orchestrating an immune response.