Which of these structures protect the ends of long bones and reduces friction at the joints?

The correct choice is cartilage, which plays a crucial role in the musculoskeletal system, particularly at the ends of long bones. Cartilage is a smooth, flexible tissue that covers the surfaces of bones at joints. Its main function is to provide a cushion between the bones, which helps to protect them from wear and tear during movement. By covering the ends of long bones, cartilage reduces friction during joint movement, allowing for smoother motion and less damage to the bones over time.

This function is especially important in high-friction joints, such as the knees and hips, where excessive rubbing of bone against bone can lead to pain and joint disorders like osteoarthritis. The presence of cartilage ensures that these joints can move freely and efficiently.

Synovial fluid, while also important in joint function, serves to lubricate the joints and reduce friction, but it is not the structure that covers and protects the bone ends. Ligaments primarily connect bones to other bones and assist in stabilizing joints, while tendons connect muscles to bones, enabling movement but not involved in the protection or cushioning of bone ends. Thus, cartilage is the key structure that fulfills the role of protecting the ends of bones and minimizing friction.