During the phase, the muscle is returning to resting length.

The correct answer is: Eccentric phase.

When talking about movement, we are talking about the activation of muscle fibers in order for this process to happen. In order for this to happen, motor neurons must activate the phases of muscle excitation, contraction and relaxation, so that the sarcomeres, and thus the fibers, can generate movement.

In essence, there are three stages, or phases, to complete muscle movement:   the concentric phase, which is when the muscle fibers are excited and begin the process of contraction. This means the muscle fibers will contract and generate movement, generating a shortening. Then we have the isometric stage, a moment in which there is no longer any more shortening of the muscle fibers, so basically, the movement is held and there is no further contraction, but neither is there relaxation of the muscle fibers. Finally, we have the eccentric phase. This phase is characterized precisely because it is the moment when the muscles begin to return to their resting position.

the answer to your question is b