What common mechanism compensates for the torque produced by a helicopter's main rotor?

The tail rotor serves as the common mechanism that compensates for the torque produced by a helicopter's main rotor. When the main rotor spins, it creates a torque that tends to rotate the helicopter's fuselage in the opposite direction. To counteract this effect and maintain directional control, helicopters are equipped with a tail rotor. The tail rotor generates thrust in the opposite direction of the main rotor's torque, allowing the pilot to maintain stable flight without the helicopter spinning uncontrollably.

The other options do not play this compensatory role. The blade flap refers to the movement of the rotor blades during flight, primarily affecting lift and maneuverability rather than torque compensation. Cyclic pitch alters the lift across the rotor disc to achieve forward motion or turns but does not directly counteract torque. Collective pitch changes the pitch angle of all the blades simultaneously to control altitude but does not address the torque reaction from the main rotor. Thus, the tail rotor is uniquely essential for balancing out the torque forces at play.