Answer:
The correct answer is Cardiac muscle.
Step-by-step explanation:
The heart muscle is a muscle that works involuntarily, and is only found in the heart. Its function is to pump the blood to the body through a contraction, which allows us to stay alive. It is also called myocardium.
The fact that this muscle works involuntarily is what it does not give us as living beings, the ability to regulate or have control over the heart rate.
Given this information, we can say that the involuntary muscle that is found only in the heart is cardiac muscle.