Final answer:
The correct answer is bursae, which are fluid-filled sacs that cushion and prevent friction in the body's joints, such as those in the leg.
Step-by-step explanation:
Closed, fluid-filled sacs that function as cushions in the body are called bursae. These bursae serve an important function in the joints by preventing friction between skin, muscle, or tendon and underlying bone, particularly in complex joints such as the one that unites the femur and tibia of the leg. Unlike neurons which are cells that transmit nerve impulses, or synapses which are the junctions between neurons, or alveoli which are the air sacs in the lungs responsible for gas exchange, bursae specifically cushion body organs and structures from damage.