Final answer:
When muscle fibers are stimulated, their membranes undergo voltage changes.
Step-by-step explanation:
When muscle fibers are stimulated, the membranes undergo voltage changes. As the cell membrane depolarizes, voltage-gated sodium channels open and sodium ions enter the muscle fiber, triggering an action potential that spreads along the entire membrane. This action potential initiates excitation-contraction coupling, leading to the release of calcium ions from the sarcoplasmic reticulum and ultimately muscle contraction. Therefore, the correct answer is c) They undergo voltage changes when stimulated.