What type of joint has grown together to form a very stable connection?

The correct answer describes a symphysis, which is a type of joint where two bones are joined together by fibrocartilage. This structure allows for limited movement between the bones while providing significant stability. An example of a symphysis is the pubic symphysis, which is found between the left and right pubic bones in the pelvis. The stability provided by a symphysis is crucial in areas of the body that require strength and support, especially during activities like walking or running where weight-bearing is involved.

In contrast, the other joint types mentioned have different structural characteristics. A ball and socket joint allows for a wide range of motion in multiple directions (like the shoulder or hip), but does not provide the same level of stability as a symphysis. A saddle joint, while allowing for movement in two planes (like in the thumb joint), is still less stable than a symphysis. A hinge joint, such as the elbow or knee, permits movement primarily in one plane but is also not as stable as a symphysis since it depends on ligaments and surrounding muscles for its integrity.