The correct answer is option 2: transfusion of contaminated blood and blood products. The transmission of HIV infection primarily occurs through certain bodily fluids, including blood. One of the most common ways HIV is transmitted is through the transfusion of contaminated blood or blood products, such as plasma or platelets. It is important to note that this form of transmission is now extremely rare in most countries due to strict testing and screening procedures for donated blood.

Options 1, 3, and 4 are not correct. AIDS is not caused by eating contaminated food and water, inhaling polluted air, or shaking hands with an infected person. HIV is not transmitted through these means. HIV is primarily transmitted through activities that involve the exchange of bodily fluids, such as unprotected sexual intercourse, sharing needles or syringes for drug use, and from mother to child during childbirth or breastfeeding.

Alert - correct answer should be option 1: unprotected sexual intercourse. Unprotected sexual intercourse is the most common mode of transmission for HIV. It is important to use condoms and engage in safe sexual practices to reduce the risk of HIV transmission.