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

Smooth muscle is the correct choice because it is a type of muscle tissue that is involuntary and found in the walls of hollow organs such as the intestines, blood vessels, bladder, and uterus. This muscle type functions without conscious control, allowing for automatic regulation of processes like digestion and blood flow. The cells of smooth muscle are non-striated, have a spindle shape, and contract slowly and rhythmically.

Cardiac muscle is also involuntary, but it is specifically found only in the heart. It shares some characteristics with skeletal muscle, such as striations, but its primary function is to pump blood throughout the body.

Skeletal muscle is voluntary muscle, meaning it is under conscious control and is responsible for movements of the skeleton. It is striated and typically attached to bones.

Voluntary muscle refers to all muscles that are consciously controlled, which includes skeletal muscle but not smooth or cardiac muscle. Therefore, the distinctive characteristics of smooth muscle make it the appropriate answer to this question, as it is the only type specifically associated with the involuntary control and the walls of internal organs.