The correct answer is A. Lymphocytes.
Lymphocytes are a type of white blood cell that play a major role in the immune system. They produce antibodies, which are proteins that help the body fight off infection.
Plasma cells are a type of B lymphocyte that produce antibodies. They are formed when B lymphocytes are activated by an antigen, which is a substance that the body recognizes as foreign.
RBCs, or red blood cells, are responsible for carrying oxygen throughout the body. They do not produce antibodies.
T-cells are a type of white blood cell that play a role in the immune system. They help to regulate the immune response and to kill infected cells. They do not produce antibodies.