What is shaped like a sac and contains a slippery fluid lining known as a bursa?

The correct answer is that a bursa is often described as shaped like a sac and contains a slippery fluid that helps to reduce friction between tissues such as bones, muscles, tendons, and ligaments. In anatomy, a bursa acts as a cushion and provides lubrication, allowing for smoother movement in joints and preventing irritation from friction.

The term "sheath" is commonly associated with structures that encase or surround organs or tissues, especially nerves or tendons, but it does not specifically refer to the sac-like structure containing the slippery fluid. The other options, like "cushion," "sleeve," and "pod," do not accurately describe the anatomical characteristics of a bursa, as they lack the specific structure or function represented by a bursa in the body. The precise definition of a bursa focuses on its role in minimizing friction, which is not captured by the other terms.