The correct answer is Flexion.

The human body moves in three primary anatomical planes: the sagittal plane, frontal plane, and transverse plane. Each plane allows specific types of joint movements.

- The sagittal plane divides the body into left and right halves and is associated primarily with movements that occur forward and backward.
- Flexion refers to the action of decreasing the angle between two body parts, such as bending the elbow or knee. This movement primarily occurs in the sagittal plane.
- Abduction (Option 2) and Adduction (Option 4) involve moving a limb away from or toward the midline of the body; both occur mainly in the frontal plane.
- Rotation (Option 3) is a turning movement around an axis and typically occurs in the transverse plane.

In summary, flexion and its opposite, extension, are the primary motions that occur in the sagittal plane, making flexion the best answer here.

Reference: Gray's Anatomy, 41st Edition, Volume 1, Chapter 9 – The Musculoskeletal System / Joint Movements, Page 85