Final answer:
The orbicularis oris muscle attaches at the labial frenum on the mandible, which helps in regulating mouth movements.
Step-by-step explanation:
The muscle that attaches at the labial frenum on the mandible is the orbicularis oris muscle. This muscle regulates what comes in and goes out of the mouth.
The labial frenulum, where the orbicularis oris attaches, is a midline fold of mucous membrane that connects the inner surface of each lip to the gum.
Additionally, the mylohyoid muscle, which forms the floor of the oral cavity, attaches to the mylohyoid lines on both sides of the mandible, but this is not directly related to the labial frenum.