A- Helper T cells are arguably the most important cells in adaptive immunity, as they are required for almost all adaptive immune responses. 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.
B- However, a small portion of long-lived T cells still remains for rapid response upon pathogen re-exposure. This kind of cells is called memory T cells. Because memory T cells have been trained to recognize specific antigens, they will trigger a faster and stronger immune response after encountering the same antigen. This is how vaccines work to protect us against infection.
C- Suppressor T cells are sensitive to high concentrations of circulating lymphokine hormones, and release their own lymphokines after an immune response has achieved its goal. This signals all other immune-system participants to cease their attack. Some memory B-cells remain after this signal to ward off a repeat attack by the invading organism.
D- A cytotoxic T cell (also known as TC, cytotoxic T lymphocyte, CTL, T-killer cell, cytolytic T cell, CD8+ T-cell or killer T cell) is a T lymphocyte (a type of white blood cell) that kills cancer cells, cells that are infected (particularly with viruses), or cells that are damaged in other ways.
So, the correct option is 'option A'.