A. Cytotoxic T-cells
B. Plasma cells
C. Natural killer cells
D. Helper T-Cells
Helper T cells are important cells that stimulate the action of other immune cells. For instance, they not only help activate B cells to secrete antibodies and macrophages to destroy ingested microbes, but they also help activate cytotoxic T cells to kill infected target cells.
Therefore, the Correct Answer is D.