Final answer:
The muscle that opposes a specific action is called the B) antagonist. It plays a critical role in maintaining positions and controlling movements, while a prime mover or agonist is the main muscle responsible for action, with synergists assisting.
Step-by-step explanation:
The muscle that opposes a particular action is known as the antagonist.
The role of an antagonist muscle is crucial in muscle function for two key reasons: firstly, they help maintain body or limb position, like when holding the arm out or standing up straight; secondly, they control rapid movements, such as those in shadow boxing or in checking the motion of a limb.
While the prime mover, or agonist, is the main muscle responsible for an action, the synergist muscles assist the prime mover. An example of a prime mover is the biceps brachii when lifting a cup, with muscles such as the brachialis acting as synergists to aid in the movement.