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

Smooth muscle is the correct answer because it is specifically characterized as involuntary and is located in the walls of hollow organs, such as the stomach, intestines, and blood vessels. This muscle type functions automatically to control the movement of substances through the organs it resides in, without conscious effort.

In contrast, skeletal muscle is voluntary, meaning it is under conscious control and is primarily responsible for body movements. Cardiac muscle, while it is also involuntary, is specifically found in the heart and not in the walls of other hollow organs. Lastly, "voluntary muscle" is a term used to describe skeletal muscle, which again emphasizes conscious control rather than the involuntary nature of smooth muscle. Thus, smooth muscle is the clear choice when considering these factors.