Which of the following is characterized as a plane (uniaxial) joint?

The coxal (hip) joint is a ball-and-socket joint, which is multiaxial and permits movement in all three planes including flexion/extension, abduction/adduction, and medial/lateral rotation.

The atlantooccipital joint is a condyloid joint formed by the occipital condyles articulating with the superior facets of the atlas, permitting biaxial movement including flexion/extension and lateral flexion.

The radiocarpal (wrist) joint is an ellipsoid (condyloid) joint classified as biaxial, allowing flexion/extension and radial/ulnar deviation but not rotation.

The sacroiliac joint is formed by the articulation of the auricular surfaces of the sacrum and ilium, which are relatively flat and allow only small gliding movements. It is heavily reinforced by the anterior and posterior sacroiliac ligaments that severely restrict range of motion, which is consistent with the functional characteristics of a plane joint - limited movement along or about a single axis compared to biaxial or multiaxial joint types.