Final answer:
Chondrichthyes are fast and maneuverable primarily due to their cartilaginous skeleton, which is lighter and more flexible than a bony skeleton, allowing for swift and agile movement in the water.
Step-by-step explanation:
The feature of Chondrichthyes that allows them to be fast and maneuverable is their cartilaginous skeleton. Unlike a bony skeleton, cartilage is lighter and more flexible, enabling these creatures to move swiftly through the water with great agility. The cartilaginous skeleton supports sharp and responsive movements, making Chondrichthyes such as sharks highly efficient predators. Paired fins, while important for stability and steering, are not unique to Chondrichthyes and so do not distinguish them in terms of maneuverability. Additionally, while fins can be supported by either cartilage or bone, it is the fundamental nature of the cartilaginous skeleton that provides the basis for their enhanced speed and maneuverability.