**Core Concept**
The question requires knowledge of the epidemiology and clinical presentation of ovarian tumors in pregnancy. Ovarian tumors are a significant concern during pregnancy due to their potential impact on both the mother and the fetus.

**Why the Correct Answer is Right**
The most common ovarian tumor in pregnancy is the **mature teratoma**. Mature teratomas are a type of germ cell tumor that typically arise from the ovary and are composed of well-differentiated tissue from all three germ layers: ectoderm, endoderm, and mesoderm. They are often benign and can grow to a significant size during pregnancy. The increase in estrogen levels during pregnancy can stimulate the growth of ovarian tumors, including mature teratomas.

**Why Each Wrong Option is Incorrect**
* **Option A:** Endometriomas are not typically associated with pregnancy and are more common in non-pregnant women.
* **Option B:** Ovarian cystadenomas are a type of epithelial tumor, but they are less common in pregnancy compared to mature teratomas.
* **Option D:** Dysgerminomas are a type of germ cell tumor, but they are more common in younger women and are not the most common ovarian tumor in pregnancy.

**Clinical Pearl / High-Yield Fact**
Mature teratomas are often asymptomatic until they reach a significant size, at which point they can cause abdominal pain, a palpable abdominal mass, or even torsion. It is essential for obstetricians to be aware of this condition and to perform regular ultrasound assessments to detect any ovarian tumors that may be present during pregnancy.

**Correct Answer:** C. Mature teratoma.