**Core Concept**
Vaginal delivery refers to the process of childbirth where the fetus is born through the birth canal, whereas cesarean delivery involves surgical incision in the abdomen and uterus. The decision to allow vaginal delivery is influenced by various factors, including fetal and maternal health, the size and position of the fetus, and any potential complications during labor.

**Why the Correct Answer is Right**
The correct answer is **B. Transverse lie with a single fetus**. A transverse lie occurs when the fetus is positioned sideways in the uterus, which can make vaginal delivery challenging or impossible. In most cases, a transverse lie requires cesarean delivery to ensure the safety of both the mother and the baby. This is because a transverse lie can cause difficulties during labor, such as a prolonged second stage of labor or a higher risk of umbilical cord prolapse.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not specified, but usually vaginal delivery is allowed in multiple gestations (twins or more) if both fetuses are in a head-down position and the mother's health allows it.
* **Option C:** Not specified, but vaginal delivery might be possible in cases of placenta previa with a low-lying placenta if the baby is in a head-down position and the mother's health allows it.

**Clinical Pearl / High-Yield Fact**
In cases of transverse lie, it is essential to monitor fetal heart rate and maternal well-being closely, as there is an increased risk of umbilical cord prolapse and fetal distress.

**Correct Answer: B. Transverse lie with a single fetus**