Final answer:
The muscle that is not involved in mastication among the options provided is the risorius, which is associated with facial expressions. The action of the masseter muscle is chewing, as it is the main muscle that elevates the mandible to close the mouth.
Step-by-step explanation:
The muscles responsible for mastication are primarily the masseter, temporalis, medial pterygoid, and lateral pterygoid. When considering the options given: A) temporalis, B) risorius, C) lateral pterygoid, and D) masseter, the muscle that is not involved in mastication is the risorius. The risorius muscle is associated with facial expressions, particularly in pulling the corner of the mouth to the side.
As for the action of the masseter muscle, the correct answer is chewing, which corresponds with option B. The masseter is a main muscle for chewing because it elevates the mandible to close the mouth, playing a critical role in crushing and grinding food.