AIDS, or Acquired Immuno-Deficiency Syndrome, is caused by a virus. The correct answer is option 2. AIDS is caused by the Human Immunodeficiency Virus (HIV), which attacks the immune system and weakens it over time. This makes the body more susceptible to infections and diseases. Option 1, protozoa, is not the cause of AIDS. Protozoa are single-celled organisms that can cause various infections, but they are not responsible for the development of AIDS. Option 3, fungus, and option 4, bacteria, are also not the cause of AIDS. Fungal and bacterial infections can occur in individuals with weakened immune systems, such as those with AIDS, but they are not the primary cause of the syndrome. It is important to understand that AIDS is a serious condition caused by a specific virus, HIV.