**Core Concept**
The transmission of human immunodeficiency virus (HIV) primarily occurs through exposure to the virus in body fluids, particularly those containing high viral loads. This highlights the importance of understanding the primary modes of HIV transmission.

**Why the Correct Answer is Right**
HIV is most commonly transmitted through sexual contact, particularly vaginal or anal intercourse. This is because these modes of transmission involve direct exposure to the virus present in the genital secretions of an infected individual. Specifically, HIV is present in high concentrations in semen, vaginal fluid, and rectal fluid, making these fluids the primary sources of transmission. The virus can also be transmitted through blood-to-blood contact, such as through sharing needles or transfusions of contaminated blood products.

**Why Each Wrong Option is Incorrect**
**Option A:** HIV transmission through saliva is extremely rare and not considered a major source of transmission. While HIV can be present in saliva, the virus is typically not transmitted through casual contact, such as kissing or sharing utensils.
**Option B:** While HIV can be transmitted through breast milk, this is not considered a major source of transmission. Mother-to-child transmission of HIV is primarily a concern during pregnancy or childbirth.
**Option C:** Feces and urine are not considered major sources of HIV transmission. While HIV can be present in these fluids, the virus is typically not transmitted through casual contact or exposure to these fluids.

**Clinical Pearl / High-Yield Fact**
It is essential to note that HIV can be transmitted through other body fluids, including blood, semen, vaginal fluid, and rectal fluid. Practicing safe sex, using condoms, and avoiding sharing needles or other equipment can significantly reduce the risk of HIV transmission.

**Correct Answer: C. Semen, vaginal fluid, and rectal fluid are the primary sources of HIV transmission.**