Ans. is 'c' i.e., Helper T cells * T-cells constitutes 60%-70% of the circulating peripheral lymphocytes. Based on their surface markers, target cells and functions the following T cell category have been identified :-A) Helper T-cells (Inducer T-cells)# These cells constitute 60% of total T-cells. These have CD4 surface marker and bind to MHC class II (MHC class II restricted). There are following types of CD4 cells (Helper T-cells) :-1) Effector cells (Effector CD4 helper T-cells)* There are divided into :-i) TH-1 cells# These are activated by IFN-y and themselves produce IL-2, IFN-y and IL-12. These are the primary cells involved in delayed hypersensitivity, cell mediated immunity, macrophage activation and killing of intracellular microbes (M. tuberculosis, M. leprae).# These cells also induce destruction of target cells by activating T-cells to become cytotoxic T-cells and by activating NK cells.ii) TH-2 cells# These are activated by IL-4 and themselves produce IL-4, IL-5, IL-6 and IL-13. These cells facilatate synthesis of all antiboides except IgG2 b. These cells induce synthesis of IgE more efficiently and cause activation of mast cells and eosinophils. Therefore these cells provide defence against Helminthic parasites.iii) TH-17 cells# These cells are powerful recruiters of neutrophils and monocytes to play a role in severe inflammatory diseases. These cells produce IL-17, IL-22 and chemokines which recruit neutrophils and monocytes. TH-17 cells produce IL-21, which amplifies the TH-17 (self) response.2) Memory cells (Memory CD4 helper T-cells)* Provide memory, i.e., retain the antigenic affinity of the originally activated T cells and are used to act as later effector cells during a second immune response.B) Cytotoxic T-cells (Cytolytic T-cells)# These cells constitute 30% of total T-cells. These cells have CD8 surface marker and are MHC class I restricted.# They kill and lyse target cells including tumor cells, virus infected cells and allograft; and participate in type II Hypersensitivity.C) Suppressor T-cells# These cells have CD8 surface marker and are MHC class I restricted. These cells down regulate immune response.