What should be done at the moment the aircraft reaches the base leg?

When the aircraft reaches the base leg of the traffic pattern, the appropriate action is to reduce power and begin the descent. This phase of flight is crucial as the pilot prepares for the final approach to the runway. Reducing power allows the aircraft to start descending to the appropriate altitude for landing while maintaining a controlled and stable flight path.

At this point in the traffic pattern, the aircraft typically transitions from a relatively high and level flight configuration to a lower altitude to align with the runway for landing. Reducing power helps manage the aircraft's speed and descent rate, ensuring that the pilot can smoothly and safely reach the final approach.

The other actions mentioned do not align with the proper procedures for the base leg. For instance, raising the landing gear is not performed until the landing sequence is further along, leveling off does not prepare the aircraft for landing, and accelerating to approach speed is usually associated with the final approach phase rather than the base leg. Thus, the best course of action at the moment the aircraft reaches the base leg is indeed to reduce power and begin the descent.