What is the function of a bursa in the skeletal system?

The correct answer highlights the primary function of a bursa as it is a small sac filled with synovial fluid. This fluid serves a critical role in reducing friction between moving structures, such as tendons and bones, within joints. By acting as a lubricant, the bursa allows for smoother movement and helps to prevent wear and tear on these structures.

The bursa's essential role in minimizing friction is particularly important in areas where tendons and muscles slide over bones during various activities, thereby protecting them from stress and potential injury. While the other options describe functions related to the skeletal system and joints, they do not accurately capture the specific nature of a bursa. Only the option that identifies it as a small sac containing synovial fluid correctly represents its anatomical and functional characteristics.