Which motion is characterized by the closing of a joint angle?

Flexion is the motion that involves decreasing the angle between two body parts, resulting in the closing of a joint angle. For instance, when you bend your elbow to bring your hand closer to your shoulder, the angle at the elbow joint decreases; this is flexion. In many joints, such as the knee or hip, bending also reduces the angle between the upper and lower parts of the leg or body.

Understanding the other terms helps to clarify why flexion is the correct choice. Extension refers to the straightening motion that increases the joint angle, effectively the opposite of flexion. Abduction denotes movement away from the midline of the body, which does not pertain to angle reduction. Lastly, adduction involves moving a limb toward the midline, again not characterized by the closing of an angle but rather bringing parts closer together. Hence, flexion is specifically defined as the action that decreases the angle at a joint, making it the most fitting answer.