Tail rotor power requirement is reduced as forward airspeed increases due to:

The correct answer is based on the phenomenon known as the keel effect. As a helicopter gains forward airspeed, the aerodynamic forces on the tail rotor change. The increased horizontal flow of air around the helicopter reduces the relative wind acting on the tail rotor, which subsequently decreases the power needed for it to counteract the torque produced by the main rotor.

The keel effect helps establish a more stable flight by creating a tendency for the helicopter to return to a straight flight path, allowing for a smooth transition as speed increases. This is especially important in maintaining control and directional stability during flight, as the tail rotor's primary function is to counteract the rotational movement of the helicopter caused by the main rotor.

Other terms like ground effect, translational lift, and translating tendency are related to different aerodynamic principles or flight characteristics but do not specifically address the reduction of tail rotor power requirement as forward speed increases in the manner that the keel effect does.