What characterizes a stall's occurrence in virtual aircraft operations?

A stall in the context of virtual aircraft operations, as well as in real-world aviation, is primarily characterized by a disruption of smooth airflow over the wing. This disruption typically occurs when the angle of attack exceeds the critical angle, causing the air to separate from the wing's upper surface. As a result, the wing is no longer able to generate sufficient lift, which can lead to a decline in performance and control of the aircraft.

In terms of the other given options, the engine stopping running does not necessarily indicate a stall; an engine failure can happen without affecting the aircraft's aerodynamic performance. Rapid loss of altitude may occur as a consequence of a stall but isn't the defining characteristic of one; instead, it is a symptom of the stall's effects on flight. Lastly, increased lift from the propeller is misleading; during a stall, lift actually decreases due to the loss of effective airflow over the wing, and while the propeller may generate thrust, it doesn’t contribute to increased lift when stalling. Therefore, the disruption of smooth airflow over the wing is the most accurate representation of what constitutes a stall in aircraft operations.