**Core Concept**
Ovarian tumors can be classified into several types, including **epithelial**, **germ cell**, and **sex cord-stromal tumors**. During pregnancy, these tumors can cause various complications due to their growth and potential to undergo **torsion** or **rupture**. The most common type of ovarian tumor to cause complications during pregnancy is related to their increased risk of **torsion**.

**Why the Correct Answer is Right**
The correct answer is related to **dermoid cysts**, a type of **mature teratoma**, which are a subset of **germ cell tumors**. These tumors are more likely to undergo **torsion** due to their **pedunculated** nature and weight, causing severe abdominal pain and potentially leading to **necrosis**.

**Why Each Wrong Option is Incorrect**
**Option A:** Is incorrect because while **epithelial tumors** are the most common type of ovarian tumor, they are not as frequently associated with complications during pregnancy as **dermoid cysts**.
**Option B:** and **Option D:** Are not specified but would likely be incorrect as they do not represent the tumor type most commonly associated with complications during pregnancy.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that **dermoid cysts** are at high risk of **torsion**, especially during pregnancy, and this complication can be severe. Thus, any pregnant woman with an ovarian mass should be carefully monitored.

**Correct Answer:** D. Dermoid cyst.