When does an airplane maintain level flight?

Level flight occurs when an aircraft maintains a constant altitude and does not climb or descend. This state is achieved when the forces acting on the aircraft are balanced. Specifically, the lift generated by the wings counteracts the weight of the aircraft, while the thrust produced by the engines balances the drag created by the aircraft's movement through the air.

In this context, having lift equal to weight means that the aircraft can remain steady in the air, neither gaining altitude nor losing it. At the same time, having thrust equal to drag ensures that the aircraft maintains a constant forward speed without accelerating or decelerating. Therefore, option B is accurate because it correctly describes the balance of forces required for level flight.

When comparing this to other options: drag exceeding lift would not allow sustained level flight, as the aircraft would descend. An increase in thrust over drag would lead to acceleration, not level flight. Finally, an aircraft cannot have zero weight in the context of flying; weight is an essential force acting on the aircraft due to gravity.