The main characteristic of osteoarthritis is the gradual breakdown of joint cartilage, which leads to joint pain, stiffness, and loss of function. This breakdown occurs over time due to various factors such as aging, joint overuse, and mechanical stress.

  
a. Osteoarthritis is primarily a non-inflammatory condition. While some inflammation may occur in later stages, it is not the main characteristic of the disease. Inflammation of the synovial membrane is more characteristic of other types of arthritis, such as rheumatoid arthritis.
b. Formation of bony nodules in the joints is not a primary characteristic of osteoarthritis. It is more commonly seen in conditions like osteophytes, which are small, bony outgrowths that can develop around the joints in response to cartilage degeneration.
d. Overproduction of synovial fluid is not a characteristic of osteoarthritis. Osteoarthritis is not primarily a disease of the synovial membrane, and excessive synovial fluid production is more characteristic of inflammatory joint conditions.