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

Smooth muscle is the correct answer because it is characterized by its involuntary function and is located in the walls of internal organs, such as the digestive tract, blood vessels, and bladder. Unlike skeletal muscle, which is voluntary and under conscious control, smooth muscle operates automatically, regulating processes like digestion and blood flow without requiring conscious thought.

This type of muscle tissue is non-striated and has a spindle-shaped appearance, which distinguishes it from skeletal (striated) and cardiac muscles. While cardiac muscle is also involuntary and found in the heart, the question specifically refers to muscles in the walls of various internal organs, making smooth muscle the most appropriate choice. Skeletal muscles are voluntary and attached to bones, and striated muscle generally refers to both skeletal and cardiac muscle, which do not fit the description provided in the question.