The correct answer is (2).
HIV that causes AIDS infects the cells of the immune system and in particular the T-helper lymphocytes, or CD4 T-cells, which are crucial to the immune system and immune response . The virus kills these cells until their number goes down gradually. It may take a few years before the decline gets to a level that the immune system is too weak to fight off infections.
The lower the count of CD4 cells becomes, the more susceptible a person becomes to viruses and infections that an otherwise healthy body would fight.