Which type of muscle is involuntary and found in the walls of internal organs?

Smooth muscle is the correct answer because it is characterized as involuntary muscle, meaning it operates without conscious control. This type of muscle is found in the walls of internal organs such as the digestive tract, blood vessels, bladder, and uterus. The cells of smooth muscle are non-striated and spindle-shaped, which allows for gradual and sustained contractions necessary for the functions of these organs, such as peristalsis in the intestines and regulating blood flow.

In contrast, striated muscle, also known as skeletal muscle, is under voluntary control and is attached to bones, enabling movement of the skeleton. Cardiac muscle, while also involuntary, is specifically found only in the heart and features an intercalated disc structure that facilitates synchronized contractions. Therefore, the unique characteristics of smooth muscle make it essential for involuntary functions in various body systems, which is why it is the correct choice for this question.