The correct answer is C) can turn without using her rudder. A twin-screw vessel is easier to maneuver than a single-screw vessel because the two propellers allow the vessel to turn without relying solely on the rudder. The counter-rotating propellers create a strong wash that can be used to pivot the vessel, effectively turning the vessel by differential thrust rather than relying on the rudder alone. This increased maneuverability is a key advantage of twin-screw vessels over single-screw designs. The other answer choices are incorrect because they do not accurately describe the primary reason why twin-screw vessels are more maneuverable. Options A and B relate to the rudder, but do not address the key advantage of using the counter-rotating propellers to turn the vessel. Option D about generating more power is not the main factor contributing to the improved maneuverability of twin-screw vessels.