Answer:
Synovial membrane.
Step-by-step explanation:
Diarthrosis may be defined as a type of synovial joint that connect the two bones lined by the cartilage. This joint is considered as the most flexible type of joint and that allows the bone movement.
The fluid that present in diarthrosis is produced by the synovial membrane known as synovial fluid. This fluid helps in lubrication and allow their movement over one another. This fluid also protect the bone and the ends are lined with articular cartilage.
Thus, the correct answer is synovial membrane.