What type of muscle is located primarily on internal organs and functions involuntarily?

The type of muscle that is primarily located on internal organs and functions involuntarily is visceral muscle, also known as smooth muscle. This muscle tissue is responsible for movements within the internal organs, such as contractions in the digestive tract, control of blood vessel diameter, and contraction of the bladder. Unlike skeletal muscle, which is under voluntary control and is attached to bones for movement of the skeleton, visceral muscle operates without conscious thought, enabling essential bodily functions to occur automatically.

Visceral muscle fibers are non-striated and spindle-shaped, distinguishing them from skeletal muscles which are striated and have a cylindrical shape. Cardiac muscle, while also involuntary, is specific to the heart and has a unique structure and function distinct from visceral muscle. Muscle fibers is a more general term that can refer to both skeletal and smooth muscle but does not specifically denote the involuntary nature or location primarily associated with internal organs. Thus, the focus on involuntary control and location within internal organs makes visceral muscle the correct answer.