Final answer:
A heart murmur occurs when there is a defect in a heart valve.
Step-by-step explanation:
The correct answer is C) a heart valve. A heart murmur occurs when there is a defect in one or more of the heart valves. Heart valves play a crucial role in regulating the flow of blood through the heart. When a valve is not functioning properly, it can result in abnormal sounds called heart murmurs.