Which type of joint is described as allowing rotation where the round end of one bone fits into a ring composed of another bone or ligament?

The type of joint that allows for rotation, where the rounded end of one bone fits into a ring formed by another bone or a ligament, is the pivot joint. This joint structure enables rotational movement around a single axis. An example of a pivot joint in the human body is found in the neck, where the first cervical vertebra (Atlas) allows the second one (Axis) to rotate, facilitating movements such as turning the head from side to side.

In comparison, hinge joints, such as those found in the elbows and knees, permit movement primarily in one plane—flexion and extension. Ball-and-socket joints, like the shoulders and hips, allow for a wide range of motion, including rotation, but the round end of a bone fits into a cup-like socket, providing multidirectional movement. Condyloid joints allow for movement but lack the degree of rotation seen in pivot joints, as they provide an oval articular surface that moves within an elliptical cavity, allowing flexion, extension, and some side-to-side motion.

Thus, the distinctive characteristic of the pivot joint—rotation around a single axis—clearly identifies it as the correct answer.