**Core Concept**
The question is testing the understanding of the primary mode of HIV transmission from an infected individual to a non-infected partner. HIV transmission occurs through the exchange of bodily fluids, such as blood, semen, vaginal and rectal fluids, and breast milk. The correct answer will highlight the specific mechanism of heterosexual transmission.

**Why the Correct Answer is Right**
HIV transmission during heterosexual intercourse primarily occurs through the direct contact of infected bodily fluids with mucous membranes. This can happen during vaginal or anal sex, and the risk is higher if the infected individual has a high viral load or if there are lesions or mucosal damage in the genital area. The virus can also be transmitted through blood, but this is less common in heterosexual transmission.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the mode of transmission. Heterosexual transmission is a specific context, and a correct answer should acknowledge this.
* **Option B:** This option is incorrect because it does not accurately describe the primary mode of transmission. While oral sex can potentially transmit HIV, it is not the most common mode of heterosexual transmission.
* **Option C:** This option is incorrect because it is too broad and does not specifically address the context of heterosexual transmission. HIV can be transmitted through various means, but the question is focused on the specific context of heterosexual intercourse.

**Clinical Pearl / High-Yield Fact**
In the context of HIV transmission, the presence of lesions or mucosal damage in the genital area increases the risk of transmission. This emphasizes the importance of safe sex practices and regular STI screenings for individuals with HIV.

**Correct Answer: D. Semen.**