Which of the following is known as a synovial joint?

The elbow is classified as a synovial joint due to its structure and function. Synovial joints are characterized by a joint capsule filled with synovial fluid, which lubricates the joint and allows for smooth movement. The elbow joint specifically is a hinge joint, enabling it to flex and extend, making it highly mobile and functional for various activities involving the arm.

In contrast, the ulna and sacrum are bones rather than joints, and the pelvis, while part of the skeletal system, is not classified as a synovial joint itself. The pelvis contains several joints, some of which are synovial, but the pelvis as a whole is a broader structure. Therefore, the elbow is the clear example of a synovial joint among the choices provided.