Active immunity development is related to
A. Natural killer cells
B. Memory cells
C. Helper T cells
D. Suppressor T cells

Hint: Active immunity is a type of immune response when an individual is exposed to antigens or foreign bodies, which in turn triggers the immune system and develops antibodies. It differs from passive immunity as in this type the antibodies are directly injected in the individual's body.

Complete answer: Active immunity is attained by a person when the individual is exposed to antigens and it leads to the induction of immunity. Antibodies are developed by the person and can be retained permanently. The immune system develops antibodies against the antigen or microbe which are developed by the memory cells of B cells or T cells. The T cell mediates cell-mediated immunity and B-cells are responsible for humoral immunity. The T-cells consist of four types of cells mainly- T-helper cells, Cytotoxic cells, memory cells, and T-suppressor cells. The humoral immunity consists of memory cells and plasma cells, the plasma cells are responsible for the development of antibodies. Memory cells are long-lived immune cells capable of recognizing foreign particles they were previously exposed to. When memory B cells reencounter their specific antigen, they proliferate and differentiate into plasma cells, which then respond to and clear the antigen.
Hence, the correct answer is option B.

Note: The T-suppressor cells are responsible for the suppression of the immune system after the antibodies are generated. The cytotoxic cells are helpful in destroying cells that have already been infected with viruses or any other pathogens. Cytotoxic T cells destroy cells by releasing a protein called granzymes.