Which process involves the action of a muscle reducing its length?

The correct answer is identified by understanding the fundamental actions of muscles. Contraction is the process where a muscle generates force and reduces its length, which is how it facilitates movement. When a muscle contracts, it pulls the attached bones closer together, resulting in movement such as lifting an arm or bending a leg.

In contrast, relaxation refers to the muscle lengthening and returning to its resting state after contraction. Abduction is the movement of a limb away from the midline of the body, which involves the muscle lengthening and not specifically reducing in length. Extension is the opposite of contraction; it involves increasing the angle between body parts, typically leading to the lengthening of the muscle. Therefore, contraction is the process that most accurately describes a muscle reducing its length during movement, making it the correct answer.