What type of muscle can be found in the walls of internal organs?

Smooth muscles are the correct choice because they are non-striated muscles that are primarily found in the walls of hollow internal organs such as the stomach, intestines, blood vessels, and bladder. These muscles are involuntary, meaning they operate automatically without conscious control, facilitating functions like digestion and circulation.

In contrast, cardiac muscles are specific to the heart and are also involuntary but have a different structure and function compared to smooth muscles. Skeletal muscles, on the other hand, are striated and under voluntary control, allowing for movements of the skeleton and body. Voluntary muscles refer to those that can be consciously controlled, primarily skeletal muscles. Therefore, smooth muscles are distinct in their location and function within internal organs, making them the correct answer.