What will the effect be if the collective is lowered while flying straight and level, assuming all else is constant?

When the collective is lowered while the helicopter is flying straight and level, the reduction in collective pitch decreases the pitch angle of the rotor blades. This action reduces the amount of lift generated by the rotor system because the rotor blades are producing less vertical thrust. Consequently, the helicopter will start to lose altitude.

As the collective pitch is decreased, the helicopter's weight begins to exceed the available lift, leading the helicopter to descend. During this descent, because the rotor blades are still moving through the air, increasing the relative wind over the rotor system can result in an increase in airspeed. As the helicopter descends, the thrust vector can cause the nose of the helicopter to pitch down, which is a typical aerodynamic response when lift is decreased relative to weight.

Thus, while the helicopter loses altitude, the increase in airspeed is a direct result of the aerodynamic forces at play in conjunction with the collective's adjustment, making this choice the most accurate reflection of the helicopter's behavior under those conditions.