Resistance to pathogens may be developed when the pathogen comes in contact with the body either voluntarily (vaccinations) or involuntarily (contracting a disease). Resistance can also be developed through the B and T cells, it involves the cells binding an antibody to the pathogen so they cannot enter a healthy cell.
The answer is d (All of the Above). While vaccination is a way of introducing attenuated pathogens into the body to invoke an immune response and memory, ocassionaly, an infection of disease can also invoke immune response and memory after healing. This immune memory is mainly through memory T and B cells.
