Final answer:
The cells that decline in number during the final phase of HIV-1 infection, when an individual has AIDS, are the helper T (TH) cells, specifically CD4+ T cells.
Therefore the correct answer is option is D) helper T (TH) cells
Step-by-step explanation:
During the final phase of HIV-1 infection, when an individual is said to have AIDS, the cells that decline in number are the helper T (TH) cells. This drop in helper T cells leads to a severely compromised immune system, making the body more vulnerable to opportunistic infections and other immune system problems.
Helper T cells are specifically CD4+ T cells, and HIV targets these cells for infection, directly killing them or making them targets for immune system attacks. A critical decline in these CD4+ helper T cells below 200 cells per microliter of blood is one of the criteria for an AIDS diagnosis.