Final answer:
The Zygomaticus Major muscle is responsible for smiling, as it is one of the muscles of facial expression that insert into the skin to produce facial movements.
Step-by-step explanation:
The muscle responsible for smiling is the Zygomaticus Major. It is one of the muscles of facial expression which are uniquely characterized by their insertion into the skin surrounding the eyelids, nose, and mouth. These muscles, including the zygomaticus major, operate differently from other muscles in the body because they move the skin to produce expressions rather than moving bones.
The orbicularis oculi is the muscle that closes the eye, not related to smiling. The frontalis muscle helps move the scalp and the eyebrows but does not contribute to the smile. Lastly, the platysma is involved in the downward movement of the mouth and the neck, which is also not directly related to smiling.