**Core Concept**
Turner's syndrome is a genetic disorder affecting females, characterized by the partial or complete absence of one X chromosome. Individuals with Turner's syndrome often exhibit various physical and developmental anomalies, including cardiovascular malformations. The most common cardiac anomaly in Turner's syndrome is of particular interest in this question.

**Why the Correct Answer is Right**
The most common cardiac anomaly in Turner's syndrome is a bicuspid aortic valve. This is due to the abnormal development of the aortic valve leaflets, resulting from the genetic defect that causes Turner's syndrome. The bicuspid aortic valve is a congenital anomaly where the aortic valve has only two cusps instead of the normal three. This can lead to aortic stenosis or regurgitation, and may increase the risk of aortic aneurysm or dissection.

**Why Each Wrong Option is Incorrect**
**Option A:** Coarctation of the aorta is a congenital anomaly where the aorta is narrowed, but it is not the most common cardiac anomaly in Turner's syndrome.
**Option B:** Ventricular septal defect (VSD) is a common congenital heart defect, but it is not specifically associated with Turner's syndrome.
**Option C:** Atrial septal defect (ASD) is another congenital heart defect, but it is not the most common cardiac anomaly in Turner's syndrome.

**Clinical Pearl / High-Yield Fact**
Individuals with Turner's syndrome are at increased risk of cardiac anomalies, and should undergo regular cardiac evaluation and monitoring to detect any potential issues early.

**Correct Answer:** A. Bicuspid aortic valve