When should the transition (power reduction) for a takeoff-flap setting be initiated?

For a takeoff-flap setting, the transition or power reduction should be initiated no sooner than when abeam the intended point of landing. This timing is crucial because it allows the pilot to maintain an appropriate approach speed and descent profile while also providing adequate glide performance to reach the landing area. Abeam the landing point typically occurs during a downwind leg in the traffic pattern, which is the proper moment to begin descending towards the runway without losing necessary altitude control.

Establishing at this point provides an effective way to manage the aircraft's energy state while also giving the pilot time to assess the approach and make any necessary adjustments before entering the final leg of the landing. This phase of the approach allows for smooth transitions in power settings, ensuring the aircraft maintains optimal flying characteristics and control.

The other options would not provide the appropriate timing or control for the approach. Initiating power reduction on final approach or while established on the glide slope could lead to challenges in managing descent and speed. Likewise, transitioning power reduction only when in the landing configuration does not allow for the necessary adjustments during the critical approach phase leading to the runway.