**Core Concept**
Transverse lie refers to the orientation of a fetus in the uterus where the baby's shoulders are aligned with the mother's spine, rather than in a head-down or feet-down position. This can be a cause of concern during pregnancy due to potential difficulties during delivery. Placenta previa is a condition where the placenta covers the cervix, which can lead to complications.

**Why the Correct Answer is Right**
Placenta previa is a common cause of transverse lie because the placenta's abnormal location can make it difficult for the fetus to move into a head-down position. The placenta's attachment to the lower uterine segment can also cause the uterus to contract, further preventing the fetus from moving into a more favorable position. This can lead to a prolonged pregnancy and increased risk of complications during delivery.

**Why Each Wrong Option is Incorrect**
**Option A:** Prolonged labor is a potential consequence of a transverse lie, but it is not a cause. Prolonged labor can occur if the fetus is unable to move into a more favorable position, but it is not the underlying reason for the transverse lie.

**Option B:** Nulliparity (never having given birth before) can increase the risk of certain pregnancy complications, but it is not directly related to the occurrence of a transverse lie.

**Option D:** Oligohydramnios (low levels of amniotic fluid) can increase the risk of certain complications during pregnancy, but it is not a common cause of transverse lie.

**Clinical Pearl / High-Yield Fact**
It is essential for healthcare providers to identify and manage placenta previa promptly to prevent potential complications, including preterm labor, fetal distress, and hemorrhage.

**✓ Correct Answer: C. Placenta previa**