Spur:
(b) is the correct answer. As we accelerate, transitional (or translational) lift increases, induced drag reduces and the induced flow decreases.
For a given collective setting (pitch angle) as you accelerate from a hover:
the induced flow will decrease
the induced angle will decrease
the induced drag will decrease (decrease in rotor drag and therefore increase in RRPM/decrease in engine power required)
the angle of attack and total rotor thrust will increase
and so the aircraft will want to climb.
If we don't want that to happen during the initial stage of takeoff, we reduce the pitch angle with collective, which will reduce the increased angle of attack and total rotor thrust back towards where it was prior to climbing.
Shawn:
As you accelerate the induced flow decreases (not increases) which reduces the induced drag. Anything which brings the Relative airflow closer to the plane of rotation will tilt the Total Reaction on the blade more vertical and reduce drag on the rotor. Typo or am I missing something?