The correct answer is A. Plasma cells.
Plasma cells are a type of B cell that produce antibodies. Antibodies are proteins that bind to specific antigens, which are foreign substances that can cause an immune response. When an antibody binds to an antigen, it can help to neutralize the antigen or signal to other immune cells to attack it.
Monocytes are a type of white blood cell that can develop into macrophages or dendritic cells. Macrophages are cells that can engulf and destroy foreign substances, while dendritic cells are cells that present antigens to T cells.
Mast cells are cells that release chemicals that can cause inflammation. Neutrophils are a type of white blood cell that can phagocytose, or engulf and destroy, bacteria.
Plasma cells are the only cells that produce antibodies.