Final answer:
Synergistic muscles that immobilize a joint are classified as fixators.
Step-by-step explanation:
Synergistic muscles that immobilize a joint are also classified as fixators (option b).
A synergist is a muscle that helps an agonist (option a) work. It assists the prime mover in performing a specific movement. However, a synergist that stabilizes the bone that is the attachment for the prime mover's origin is called a fixator (option b).
Antagonists (option d) are muscles with the opposite action of the prime mover. They oppose or resist the movements of the agonists. Prime movers, as mentioned earlier, are the principal muscles involved in an action (option c).