After an HIV infection, the body of the patient starts becoming immun…

The correct answer is Option (3) → Progressive decline in the number of helper T- lymphocytes

Option (1) The damage caused to secondary lymphoid organs — Incorrect. HIV does not work by destroying lymph nodes or the spleen. It attacks particular cells of the immune system.

Option (2) Rapid multiplication of the virus outside the host cells — Incorrect. A virus can multiply only inside a host cell. HIV multiplies inside macrophages and helper T-cells.

Option (3) Progressive decline in the number of helper T- lymphocytes — Correct. HIV enters helper T-cells, multiplies and kills them, and the new viruses attack more helper T-cells. As their number keeps falling, the body’s immune response weakens.

Option (4) Infection by a number of pathogens — Incorrect. Repeated infections by bacteria, viruses, fungi and parasites are the result of immunodeficiency, not its cause.

NCERT Extract:
AIDS – “… After getting into the body of the person, the virus enters into macrophages where RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase. This viral DNA gets incorporated into host cell’s DNA and directs the infected cells to produce virus particles …. The macrophages continue to produce virus and in this way acts like a HIV factory. Simultaneously, HIV enters into helper T-lymphocytes (TH), replicates and produce progeny viruses. The progeny viruses released in the blood attack other helper T-lymphocytes. This is repeated leading to a progressive decrease in the number of helper T-lymphocytes in the body of the infected person.”