Which speed ensures safety by designating the upper limit of aircraft performance?

The answer identifies "Never exceed speed" as the correct choice because this speed, often marked as VNE, is critical in maintaining the integrity and safety of the aircraft during flight. It represents the maximum speed that should not be exceeded under any circumstances to avoid structural failure or loss of control. Flying at or above this speed can lead to dangerous aerodynamic conditions, such as control surface flutter or other potentially catastrophic scenarios.

While the other options play significant roles in operational safety, they do not serve the specific purpose of defining a limit that should never be surpassed. Maximum flight speed refers generally to the operational capabilities of the aircraft, which can vary by flight phase and does not encompass the safety measures dictated by VNE. Maximum landing gear extension speed relates directly to the safe operation of the landing gear but is not a universal limit for all flight conditions, and design flap speed applies specifically to configurations related to flap settings during flight rather than an overall performance limit. Thus, the never exceed speed is uniquely positioned as a fundamental safety threshold in aviation.