The most common cause of death in patients with untreated hypertension is:

The correct answer is D. congestive heart failure.

Untreated hypertension (high blood pressure) can lead to various complications, and the most common cause of death is congestive heart failure (CHF). CHF occurs when the heart muscle becomes weakened due to chronic high blood pressure, leading to impaired pumping function and fluid buildup in the lungs and other parts of the body.