Final answer:
The muscle involved in producing horizontal wrinkles in the forehead is the frontal belly of the epicranius, also known as the frontalis muscle.
Step-by-step explanation:
The muscle involved in producing horizontal wrinkles in the forehead is the frontal belly of the epicranius. This muscle is also known as the frontalis muscle. When the frontal belly contracts, it pulls the skin of the forehead upward and backward, which creates horizontal wrinkles. The muscles zygomaticus major, medial pterygoid, and temporalis are not associated with this action. The zygomaticus major is primarily responsible for pulling the mouth corners upward when smiling, the medial pterygoid helps in the act of chewing (mastication), and the temporalis assists in closing the jaw.