Final answer:
Smooth muscle is the only type of muscle that is both nonstriated and involuntary, performing automatic functions in the body.
Step-by-step explanation:
The only muscle type that is nonstriated and involuntary is smooth muscle. Unlike skeletal and cardiac muscle, which both have a striated appearance due to their arrangement in bundles, smooth muscle has a nonstriated structure as its cells are arranged in sheets. Additionally, smooth muscle is controlled involuntarily, which means it operates without conscious intent, functioning in various parts of the body such as the digestive system and blood vessels to regulate processes like digestion and blood flow.