Final answer:
The process of aging in flexibility starts with reduced collagen solubility, then increased tendon rigidity, and lastly decreased joint lubrication.
Step-by-step explanation:
The process of aging in flexibility starts with reduced collagen solubility, which leads to increased tendon rigidity. The last stage in this process is decreased joint lubrication, which contributes to reduced flexibility in the joints.
For example, as collagen decreases in some connective tissues, the tendons become less flexible and more rigid. This can affect the range of motion in joints and make them stiffer. Therefore, the correct answer is a) Decreased joint lubrication.