Final answer:
Heterosexual contact has a d) variable risk for HIV transmission, influenced by factors such as sexually transmitted co-infections and viral load. Consistent condom use greatly reduces this risk.
Step-by-step explanation:
Heterosexual contact has variable risk depending on other factors. While most HIV transmissions globally occur through heterosexual contact, the risk varies depending on factors such as the presence of other sexually transmitted infections, particularly with open sores on the genitals. An increased risk is also associated with higher viral loads, especially during the early months of infection.
Consistent use of condoms significantly reduces the risk of HIV transmission. It's important to note that HIV transmission can only occur between individuals where one is already infected with the virus; it cannot be spread by uninfected individuals or through casual nonsexual contact.