Which of the following is classified as a "ball and socket" joint?

The hip joint is classified as a "ball and socket" joint due to its unique anatomical structure that allows for a wide range of motion. This type of joint consists of a rounded head of one bone, in this case, the femur, that fits into a cupped socket of another bone, specifically the acetabulum of the pelvis. The design enables rotational movement as well as flexion, extension, abduction, and adduction, making it one of the most mobile joints in the human body.

In contrast, the other joints listed serve different functions and lack the specific characteristics of a ball and socket configuration. For example, the stifle joint, commonly referred to as the knee in quadrupeds, functions more like a hinge joint, allowing movement primarily in one plane. The elbow joint also operates as a hinge joint, permitting flexion and extension, but with limited rotational ability. Lastly, the atlantoaxial joint facilitates rotation of the head but does not accommodate the range of motion seen in ball and socket joints; rather, it's a pivot joint that allows one bone to rotate around another. This structure makes the hip joint distinct in its capability for movement in multiple directions.