Which type of muscle tissue functions involuntarily in internal organs?

Smooth muscle is the correct choice because it is the type of muscle tissue that operates involuntarily and is primarily found in the walls of internal organs, such as the intestines, blood vessels, and bladder. This type of muscle facilitates essential functions like digestion and the regulation of blood flow without conscious control.

Skeletal muscle, in contrast, is under voluntary control and is responsible for movement of the skeleton. Cardiac muscle, while also involuntary, is specialized for the heart and is distinct from smooth muscle due to its unique structure and function specific to pumping blood. Elastic muscle is not a recognized category of muscle tissue; true muscle is categorized into skeletal, cardiac, and smooth types. Therefore, smooth muscle is the tissue that fits the description of functioning involuntarily in various internal organs.