Stress on an airplane's structure will occur when:

The correct understanding here revolves around how forces interact with an airplane's structure during various phases of flight. Stress on an airplane's structure fundamentally occurs when external forces act upon it, leading to a deflection from its normal, straight-and-level flight path.

In straight flight, the aircraft is in a state of equilibrium where the lift balances the weight, and thrust balances drag, resulting in no net forces that would cause stress. However, as any force deflects the aircraft from this stable state—whether through maneuvers, turbulence, or variations in speed—the structure of the aircraft experiences stress as it responds to those changes. This stress can manifest due to the need to handle altered aerodynamic loads or structural strains, which are induced by changes in configuration, direction, or speed.

The conditions described in the other choices do not inherently lead to structural stress in the same direct way. For instance, straight flight does not induce stress, and while high-speed maneuvers certainly can cause stress, the key factor is the deviation from straight flight itself, which encompasses a broader range of actions and conditions. Similarly, deploying the landing gear may introduce some aerodynamic changes but does not necessarily equate to stress compared to the comprehensive response triggered by any force causing deflection