**Core Concept**
The vertical transmission of HIV refers to the transmission of the virus from mother to child during pregnancy, childbirth, or breastfeeding. This occurs due to the presence of the virus in the mother's blood, vaginal secretions, and breast milk, which can infect the baby.

**Why the Correct Answer is Right**
The possibility of vertical transmission of HIV is significantly high if no prophylaxis is given. This is because the virus can be transmitted through the placenta during pregnancy, and the baby can also be infected during delivery through exposure to the mother's blood and vaginal secretions. The risk of transmission is estimated to be around 15-20% without any intervention. The use of antiretroviral therapy (ART) and other prophylactic measures can significantly reduce this risk.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the risk of vertical transmission of HIV can be significantly reduced with the use of antiretroviral therapy (ART) and other prophylactic measures, such as elective cesarean delivery and avoidance of breastfeeding. The Centers for Disease Control and Prevention (CDC) recommend that all pregnant women with HIV receive ART to reduce the risk of transmission to the baby.

**Correct Answer:** D. 20%