Which muscle is primarily responsible for opposing lip retraction (spreading)?

The orbicularis oris muscle is indeed primarily responsible for opposing lip retraction. This muscle is a complex set of muscles that encircles the mouth and is crucial for various functions, including articulation, mastication, and facial expressions. Specifically, it allows for the closing and puckering of the lips, counteracting muscles that may pull the lips apart or retract them.

In contrast, the other muscles listed have different roles. The risorius typically helps in pulling the corners of the mouth back, contributing to a smile or a grimace, and thus facilitates lip retraction rather than opposing it. The levator labii superioris assists in elevating the upper lip, which can also lead to a spreading or retraction effect. The zygomaticus major is involved in smiling and elevating the corners of the mouth, which, again, does not work against lip retraction. Therefore, the orbicularis oris stands out as the muscle specifically designed to bring the lips together and prevent them from retracting.