**Core Concept**
Congenital heart diseases (CHD) can significantly impact maternal health during pregnancy, particularly due to the increased risk of cardiac complications. The severity of CHD varies widely, and some conditions pose a higher risk to maternal mortality than others.

**Why the Correct Answer is Right**
The correct answer is Ebstein's anomaly. This congenital heart defect is characterized by an abnormal tricuspid valve and right ventricular hypoplasia. The tricuspid valve is abnormally formed, leading to severe tricuspid regurgitation, which can cause significant right ventricular dilation and dysfunction. This, in turn, can increase the risk of maternal mortality during pregnancy due to the increased workload on the right heart.

**Why Each Wrong Option is Incorrect**
* **Option A:** Tetralogy of Fallot is a cyanotic heart disease, but it is not typically associated with the highest maternal mortality rate compared to Ebstein's anomaly.
* **Option B:** Ventricular septal defect (VSD) is a common congenital heart defect, but it is usually not as severe as Ebstein's anomaly in terms of maternal mortality risk.
* **Option C:** Transposition of the great arteries (TGA) is a serious congenital heart defect, but it typically requires surgical correction in infancy, which reduces the risk of maternal mortality during pregnancy.

**Clinical Pearl / High-Yield Fact**
Ebstein's anomaly is often associated with maternal mortality due to the increased risk of cardiac complications during pregnancy. It is essential for obstetricians and cardiologists to carefully evaluate and manage patients with Ebstein's anomaly to minimize this risk.

**Correct Answer:** C.