Final answer:
The muscle that causes an action is known as the agonist. Synergists are muscles that help the agonist, while antagonists have the opposite function, often responsible for controlling movements or maintaining positions.
Step-by-step explanation:
The muscle that causes an action is the C. agonist. Agonists, or prime movers, are responsible for the main action during a muscle movement. For instance, lifting a cup principally involves the biceps brachii, making it the agonist.
Muscles that aid agonists, such as the brachialis in the arm which assists the biceps brachii, are known as synergists. Moreover, synergists can also serve as fixators when they stabilize the bones that the prime mover's origin attaches to.
Conversely, a muscle that has the opposite action of the prime mover is known as an antagonist. These muscles play a crucial role in maintaining body or limb positions and controlling the speed of movements.