Answer:
A. Before the plane reaches the airfield.
Step-by-step explanation:
The pilot should drop the bomb before the plane reaches the airfield. This is because the bomb will travel at the same horizontal velocity (due north) as the plane when it is falling towards the airfield. So if the pilot releases the bomb over or after the airfield, the bomb will travel north and miss the airfield.
But if the pilot drops the bomb before the airfield, it will travel north and hit its target when it falls.