**Core Concept**
Cardiac disease in pregnancy can be life-threatening, and the most common heart disease associated with maximum mortality is typically a condition that compromises cardiac output and increases the risk of cardiac failure, arrhythmias, and thromboembolic events.

**Why the Correct Answer is Right**
The correct answer is Cardiac Hypertrophic Obstruction, specifically Hypertrophic Cardiomyopathy (HCM). HCM is characterized by an abnormal thickening of the heart muscle, which can lead to obstruction of blood flow, particularly during pregnancy when cardiac output increases. This can result in symptoms such as chest pain, shortness of breath, and syncope, and can increase the risk of cardiac complications, including cardiac arrest and sudden death.

**Why Each Wrong Option is Incorrect**
**Option A:** **Rheumatic Heart Disease** is a common cardiac disease in developing countries, but it is not typically associated with maximum mortality in pregnancy.

**Option B:** **Mitral Valve Prolapse** is a common condition, but it is usually not severe enough to cause significant cardiac complications in pregnancy.

**Option C:** **Cardiac Arrhythmias** can be a concern in pregnancy, but they are not typically associated with maximum mortality.

**Clinical Pearl / High-Yield Fact**
Hypertrophic Cardiomyopathy is the leading cause of sudden cardiac death in young athletes and is also the most common cause of cardiac-related death in pregnancy.

**Correct Answer: C. Cardiac Hypertrophic Obstruction, specifically Hypertrophic Cardiomyopathy**