**Core Concept**
The question tests the understanding of HIV transmission modes, specifically the routes through which the virus can be transmitted from an infected person to another individual. HIV transmission primarily occurs through bodily fluids, such as blood, semen, vaginal fluids, and breast milk.

**Why the Correct Answer is Right**
The correct answer will be the option that represents a mode of HIV transmission that is not seen or is significantly reduced. HIV is not transmitted through casual contact, such as shaking hands, hugging, or sharing food and drinks. This is because the virus is not present in sufficient quantities in these bodily secretions to cause infection. However, HIV can be transmitted through other routes like blood transfusions, needle sharing, and from mother to child during pregnancy, childbirth, or breastfeeding.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because HIV can be transmitted through blood transfusions if the blood is contaminated with the virus.
**Option B:** This option is incorrect because HIV can be transmitted through needle sharing or other forms of injecting drug use equipment sharing.
**Option C:** This option is incorrect because HIV can be transmitted from mother to child during pregnancy, childbirth, or breastfeeding.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that HIV transmission can be significantly reduced or prevented through the use of antiretroviral therapy (ART), safe needle practices, and safe sex measures, including the use of condoms.

**Correct Answer: D. Casual contact.