Final answer:
Mucin is the protein responsible for the viscosity of normal joint fluid, creating a lubricating gel that reduces friction in synovial joints. The correct option is c.
Step-by-step explanation:
Normal joint fluid is very viscous because it contains a protein called mucin.
Mucin is heavily glycosylated by glycosaminoglycans (GAGs), which are the sugars that attach to proteins to form proteoglycans.
GAGs such as chondroitin sulfate and hyaluronic acid occupy large amounts of space, form hydrated gels, and resist compression, making the synovial fluid in joints an effective lubricant. This viscous fluid serves to reduce friction between the articular cartilage of synovial joints during movement. Option c. is the correct one.