In response to a stimulus, do skeletal muscle fibers always contract to 25% of their potential ability, always contract to 50% of their potential ability, always contract to 100% of their potential ability, or none of the above?
100% of their potential ability.
The reason for this is that contraction of skeletal muscles is governed by "All or None" principle. It supports that the degree of contraction doesn't depend on intensity of stimulus. If a stimulus of minimum intensity but capable to evoke a response known as threshold stimulus would be applied, then either all the muscle fibers will participate in contraction(100%) or none(0%) will participate.