Explanation: B cells are part of the adaptive immune system and are responsible for antibody-mediated immunity. They do not directly attack and kill infected cells. Instead, B cells produce antibodies that can recognize and neutralize pathogens or foreign substances.

Incorrect choices:

b. B cells are involved in antibody-mediated immunity, not cell-mediated immunity. They produce antibodies that circulate in the bloodstream and bind to antigens on pathogens, marking them for destruction by other immune cells.

c. Helper T cells, not B cells, primarily recognize antigens presented by antigen-presenting cells like macrophages and dendritic cells.

d. Phagocytosis of pathogens is mainly carried out by macrophages and neutrophils, not B cells.