What fluid acts as a joint lubricant and a nutrient supplier for cartilage within the joint?

Synovial fluid is the correct answer because it serves a crucial dual role in joints. It acts as a lubricant, reducing friction between the articular cartilage surfaces during joint movement, which is essential for smooth motion and functionality. In addition to lubrication, synovial fluid also supplies nutrients to the cartilage, which is avascular (lacking blood vessels) and relies on this fluid for nutrient exchange and waste removal. The composition of synovial fluid, which includes hyaluronic acid and lubricin, contributes to its viscous nature, enhancing its lubricating properties.

Other options do not fulfill both roles in the context of joint function. Cerebrospinal fluid primarily protects the brain and spinal cord and is not involved in joint lubrication or cartilage nutrition. Interstitial fluid is found between cells and facilitates nutrient exchange at the tissue level but is unrelated to joint mechanics. Plasma is the liquid component of blood, primarily involved in transporting nutrients, hormones, and waste, rather than providing lubrication or nutrients to cartilage in joints.