Which movement primarily involves decreasing the angle between body parts?

Flexion is the movement that primarily involves decreasing the angle between body parts. This action commonly occurs in joints such as the elbow and knee, where components of the body, such as the forearm and thigh, move closer together. For example, when you bend your elbow, the angle formed at the joint decreases, leading to flexion.

In contrast, other movements like extension increase the angle between body parts, such as straightening the elbow after flexion. Rotation involves turning around an axis, which does not pertain to changes in the angle between body parts. Adduction moves a limb toward the midline of the body, which also does not specifically involve angle reduction, but rather a lateral movement towards the center. Thus, flexion remains the correct choice for the action that decreases the angle between body parts.