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

Smooth muscle is the correct answer because it is the type of muscle that is involuntary and located in the walls of hollow organs such as the intestines, bladder, and blood vessels. This type of muscle is responsible for the involuntary movements that occur within these organs, like the contraction and relaxation that facilitate processes such as digestion and the regulation of blood flow. Unlike skeletal muscle, which is under voluntary control and is attached to bones for body movement, smooth muscle works automatically without conscious effort, making it essential for the functioning of various internal organs. Cardiac muscle, while also involuntary and found in the heart, does not meet the requirement of being in the walls of other organs. Fibrous muscle is not a recognized category in the classification of muscle types.