**Core Concept**
Choriocarcinoma is a rare, aggressive form of cancer that arises from the trophoblastic cells of the placenta. Its incidence is closely linked to the type of pregnancy or reproductive event that preceded it.

**Why the Correct Answer is Right**
The correct answer, spontaneous abortion, is associated with a higher incidence of choriocarcinoma because incomplete or missed abortions can lead to the persistence of trophoblastic tissue, which has the potential to undergo malignant transformation. This is due to the fact that the trophoblastic cells, which are responsible for implantation and placental development, are sensitive to the hormonal changes that occur during pregnancy. When a pregnancy is terminated prematurely or incompletely, the trophoblastic cells may not undergo the normal process of differentiation and regression, leading to an increased risk of cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** Ectopic pregnancy is not typically associated with an increased risk of choriocarcinoma, as the trophoblastic cells are usually not able to establish a normal connection with the maternal circulation.

**Option C:** Normal delivery is not a risk factor for choriocarcinoma, as the trophoblastic cells undergo normal differentiation and regression after delivery.

**Option D:** Cesarean section is a delivery method and not a risk factor for choriocarcinoma.

**Clinical Pearl / High-Yield Fact**
Choriocarcinoma is a type of gestational trophoblastic disease (GTD) that can occur after any type of pregnancy, but it is most commonly associated with molar pregnancies and spontaneous abortions. Early diagnosis and treatment of GTD are critical to prevent the development of choriocarcinoma and other complications.

**✓ Correct Answer: B. Spontaneous abortion**