What is the primary function of synovial membranes?

The primary function of synovial membranes is to lubricate joints with synovial fluid, making this the correct answer. Synovial membranes are specialized connective tissue that lines the cavities of synovial joints—such as the knees, elbows, and shoulders. These membranes secrete synovial fluid, which is essential for reducing friction between the articulating surfaces of the bones during movement. This lubrication not only facilitates smooth movement but also helps to nourish the cartilage in the joints and provides cushioning to prevent damage during physical activity.

The other options are related to different functions in the body and do not pertain to the role of synovial membranes. For example, the option about lining external surfaces pertains more to epithelial tissues, while the protective role of the brain and spinal cord identifies the function of the meninges. The absorption of nutrients is a function associated with the gastrointestinal tract, not the synovial membranes. Each of these other choices describes distinct anatomical structures with their own specific purposes.