In old age, stiffness of joints is due to the hardening of bones, inefficiency of muscles, decrease in synovial fluid, or enlargement of bones?

Synovial fluid is produced by the synovial membrane that lubricates the joint. The stiffness of the synovial membrane with aging renders it less able to produce the synovial fluid and thereby, causes stiffness of joints. It is also caused by inflammation, usually occurs or is worse immediately after awakening or after prolonged resting or immobility. Stiffness is common with arthritis. So, the correct answer is 'Decrease in synovial fluid'.