**Core Concept**
The question is testing the understanding of ovarian tumor characteristics and their propensity for torsion, particularly in the context of pregnancy. Torsion occurs when the ovary twists around its stalk, cutting off blood supply and leading to tissue necrosis. This is a medical emergency that requires prompt diagnosis and treatment.

**Why the Correct Answer is Right**
During pregnancy, the uterus expands and can cause the ovaries to move, increasing the risk of torsion. Dermoid cysts (mature teratomas) are a type of ovarian tumor that is more prone to torsion due to their size, mobility, and tendency to have a long mesosalpinx. This allows them to twist more easily, making them a higher risk for torsion during pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Dysgerminomas are a type of germ cell tumor that are not typically associated with a high risk of torsion.
**Option B:** Endometriomas are cystic structures filled with endometrial tissue and are not characteristically prone to torsion.
**Option C:** Serous cystadenomas are a type of epithelial tumor that are generally not associated with a high risk of torsion.

**Clinical Pearl / High-Yield Fact**
Dermoid cysts are the most common type of ovarian tumor and are often asymptomatic until they twist or rupture. A high index of suspicion is required for early diagnosis and treatment to prevent tissue necrosis and preserve ovarian function.

**Correct Answer:** C. Dermoid cyst.