Final answer:
The correct answer is that the total work done by gravity on the plane is zero because the plane starts and ends at sea level, meaning gravity does both negative work as the plane ascends and positive work as it descends, which cancel each other out.
Step-by-step explanation:
When we discuss the work done by forces on an object, it's important to consider the direction of the force relative to the direction of the object's displacement. For the plane's thrusters, they do positive work because they exert force in the direction of the plane's motion during takeoff and in-flight travel. However, the question suggests considering the total trip, from takeoff to landing, which means after the initial work to lift the plane (requiring force in the opposite direction of gravity), the thrusters work primarily to move the plane horizontally, not vertically.
Gravity, on the other hand, is always exerting a downward force. As the plane rises, work done by gravity is negative because gravity acts opposite to the displacement (upward motion). When the plane descends, gravity does positive work. If we consider the complete trip, from takeoff to landing at sea level, the total vertical displacement of the plane is zero, meaning gravity does zero net work, because the positive work done while descending cancels out the negative work done while ascending.
Given these considerations, the correct answer is: d. The total work done by gravity on the plane is zero.