Opposition is the movement of any bone in the body that opposes another bone in the body.

Choice A reason: Smooth and cardiac muscle are capable of contracting without direct nervous stimulation. Cardiac muscle has intrinsic pacemaker cells that generate rhythmic contractions, and smooth muscle can respond to chemical signals, stretch, and local factors independently of neural input.

Choice B reason: Skeletal muscle requires nervous stimulation to contract. It is under voluntary control and cannot initiate contraction on its own, making this choice incorrect.

Choice C reason: While cardiac muscle can contract independently, this choice omits smooth muscle, which also has autonomous contractile ability. Therefore, it is incomplete.

Choice D reason: Skeletal muscle is entirely dependent on neural input for contraction. It cannot contract without stimulation from motor neurons.

Choice E reason: Smooth muscle can contract without nervous stimulation, but this choice excludes cardiac muscle, which also shares this property. Thus, it is not the best answer.

Choice A reason: The prime mover, or agonist, is the muscle primarily responsible for producing a specific movement, not its opposite.

Choice B reason: A synergist assists the prime mover by adding force or reducing unnecessary movement. It does not produce the opposite action.

Choice C reason: Agonist is another term for prime mover. It initiates the movement, not the opposing one.

Choice D reason: Antagonist is the correct answer. It is the muscle that opposes the action of the agonist. For example, the triceps brachii is the antagonist to the biceps brachii during elbow flexion.

Choice E reason: A fixator stabilizes the origin of the prime mover so that it can act more efficiently. It does not produce opposing movement.