For sustained flight, lift must be equal to or greater than what?

In order for an aircraft to maintain sustained flight, the lift generated by its wings must be equal to or greater than the weight of the object in flight. This principle is fundamental to the physics of flight, as lift counteracts the force of gravity acting on the aircraft. When lift equals weight, the aircraft can fly at a constant altitude; if lift exceeds weight, the aircraft will ascend; and if lift is less than weight, the aircraft will descend.

Considering the other options, the speed of descent pertains more to the rate at which an aircraft is losing altitude, which is not directly related to the condition for achieving sustained level flight. The thrust produced is important for forward motion and overcoming drag but does not replace the need for lift to counteract weight. Drag represents the aerodynamic resistance that acts against the aircraft's motion but is not a determining factor for achieving lift sufficient for sustained flight. Therefore, the correct understanding of flight dynamics centers around the relationship between lift and weight.