**Core Concept**
The transmission of HIV (Human Immunodeficiency Virus) is primarily influenced by various factors including the mode of exposure, geographical location, and socio-economic conditions. Understanding the mode of transmission is crucial for public health efforts to prevent the spread of the disease.

**Why the Correct Answer is Right**
The correct answer is A. Heterosexual transmission. According to the Joint United Nations Programme on HIV/AIDS (UNAIDS), the majority of new HIV infections worldwide occur through heterosexual contact. This is attributed to the high prevalence of HIV among women in certain regions, particularly in sub-Saharan Africa, where women are more likely to engage in high-risk behaviors due to socio-economic factors. The virus is transmitted through bodily fluids, including semen, vaginal and rectal fluids, and breast milk.

**Why Each Wrong Option is Incorrect**
**Option B:** While homosexual transmission is a significant mode of HIV transmission, particularly among men who have sex with men (MSM), it is not the most common mode globally. This option is incorrect due to the higher prevalence of heterosexual transmission worldwide.

**Option C:** IV-drug abuse is a significant risk factor for HIV transmission, particularly in regions with high prevalence of injecting drug use. However, it is not the most common mode of transmission worldwide. This option is incorrect due to the lower prevalence of IV-drug abuse compared to heterosexual transmission.

**Option D:** Contaminated blood products were a major mode of HIV transmission in the early days of the epidemic, particularly in the 1980s. However, with the implementation of stringent blood screening measures, this mode of transmission has significantly decreased. This option is incorrect due to the rarity of contaminated blood products as a mode of HIV transmission.

**Clinical Pearl / High-Yield Fact**
It is essential to note that HIV transmission can occur through any bodily fluid that contains the virus, including saliva, sweat, and tears. However, the risk of transmission through these fluids is extremely low.

**✓ Correct Answer: A. Heterosexual**