Involuntary muscle found inside many internal organs.

• A. Skeletal muscle
• B. Cardiac muscle
• C. Diaphragm
• D. Smooth muscle

Answer: (D)

Involuntary muscle types and where they are found. Smooth muscle is the one that fits here because it forms the walls of many internal organs and blood vessels, controlling movements such as peristalsis in the digestive tract and changes in vessel diameter. These contractions are not under conscious control, hence the term involuntary. Smooth muscle cells are spindle-shaped, non-striated, and work slowly and steadily, coordinated by the autonomic nervous system.

Skeletal muscle, in contrast, is voluntary and attaches to bones to enable movement. The diaphragm is a skeletal muscle that aids breathing, and while it can be influenced reflexively, it’s not the smooth muscle lining internal organs. Cardiac muscle makes up the heart and is also involuntary, but it isn’t found in the walls of most internal organs.