What type of muscle operates under involuntary control?

The correct choice identifies smooth muscle as the type of muscle that operates under involuntary control. Smooth muscle is found in the walls of hollow organs such as the intestines, blood vessels, and the bladder, where it manages bodily functions without conscious effort. This involuntary control allows for automatic and rhythmic contractions necessary for processes like digestion and blood circulation.

Cardiac muscle, while also involuntary, is specialized and found only in the heart. Striated muscle generally refers to skeletal muscle, which operates under voluntary control and is responsible for movements of the skeleton. This distinction highlights that while cardiac muscle is indeed involuntary, the broader classification of smooth muscle encompasses all involuntary muscles, making it the most accurate answer to the question.