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

Smooth muscle is the correct answer because it is the type of muscle that operates involuntarily and is primarily located in the walls of internal organs such as the intestines, bladder, and blood vessels. This muscle type plays a critical role in various autonomic processes, such as digestion and regulation of blood flow, by contracting and relaxing without conscious control.

Smooth muscle cells are non-striated and have a spindle shape, which allows them to contract in a coordinated fashion to manage the movement of substances through different organ systems. Unlike skeletal muscle, which is under voluntary control and involved in movement of bones, and cardiac muscle, which is also involuntary but found exclusively in the heart, smooth muscle serves an essential function in the body’s automatic functions.

The other choices refer to different types of muscle: skeletal muscle is striated and under voluntary control, cardiac muscle is specialized for the heart and also involuntary, but smooth muscle is uniquely suited for the needs of internal organs. Thus, smooth muscle is specifically identified by its involuntary nature and location in the walls of hollow organs.