The correct answer is option 3 - T cells in blood.

AIDS is caused by the Human Immunodeficiency Virus (HIV), which attacks and destroys a type of white blood cell called T lymphocytes or T cells. These cells play a crucial role in the immune system, as they are responsible for recognizing and destroying foreign invaders, such as viruses and bacteria.

By targeting and destroying T cells, HIV weakens the immune system, making the body more susceptible to various infections and diseases. As a result, individuals with AIDS have a compromised immune system and may experience frequent infections and illnesses that would not typically affect someone with a healthy immune system.

It is important to note that HIV primarily affects T cells in the blood, not other components such as red blood cells (RBCs) or haemoglobin.