Enables quick and efficient response to secondary exposure to antigen. Absence results in no immune response. Forms antibody producing cells. Kills cancer cells and virus infected body cells. Slows or stops the immune response.
A. Cytotoxic T cell
B. Regulatory T cell
C. Memory cell

Some of the options are missing. The complete question is as following:

Enables quick and efficient response to secondary exposure to antigen. Absence results in no immune response. Forms antibody producing cells. Kills cancer cells and virus infected body cells. Slows or stops the immune response.

A. Cytotoxic T cell

B. Regulatory T cell

C. Memory cell

D. Helper T cell

E. B cell

C. Memory cell

D. Helper T cell

E. B cell

A. Cytotoxic T cell

B. Regulatory T cell

Explanation:

Memory cell are produced by B cell which remember the same pathogen for faster and efficient antibody production to secondary exposure to  antigen.

Helper T cell are required to activate B cell for antibody generation and  help activate cytotoxic T cells to kill infected target cells. Absence of Helper T cell will not allow the activation of B cell or cytotoxuc cell, so there will be no immune response.

B cell function in the humoral immunity component of the adaptive immune system responsible to secret antibody against the pathogens.

Cytotoxic T cells responsibel for their killing role by releasing two types of preformed cytotoxic protein: granzymes, which induces apoptosis in any type of target cell, and pore-forming protein perforin that froms holes in the target cell membrane which allows the entry of granzymes. Using the mechanism, cytotoxic T cell kills cancer cells and virus.

Regulatory T cells maintain homeostasis and self-tolerance by suppressing the immune response.

Hence, the correct order of functions perfomred by the cells is:

C. Memory cell

D. Helper T cell

E. B cell

A. Cytotoxic T cell

B. Regulatory T cell

layer 3 is younger than layer 6.