Final answer:
The sac-like fluid-filled structures that reduce friction between moving body parts are called bursae. These are not to be confused with synovial joints, which also contain synovial fluid but serve as points of articulation between bones rather than as friction-reducing sacs.
Step-by-step explanation:
The sac-like fluid-filled structures that reduce friction between moving parts in the body are known as bursae. These structures are found throughout the body and serve to decrease the friction experienced by tissues such as skin, muscle tendons, and ligaments as they glide over bone or muscle. Bursae contain a lubricating fluid and can be located in areas where there is likely to be a lot of movement or pressure, such as around the joints like the knee or elbow.
It's important to note that while synovial joints are related due to their feature of synovial fluid, which also reduces friction and allows for smooth movements of bones within the joint cavity, they are not the answer to this specific question. The choice here is bursae because they are the distinct sac-like structures separate from the joints themselves that aid in reducing friction. Tendons and ligaments, the other options provided, are types of connective tissues that respectively connect muscle to bone and bone to bone, but do not act as fluid-filled sacs to reduce friction.