Operating at any airspeed below VMC in a multi-engine airplane with one engine inoperative will lead to what?

Operating at any airspeed below VMC (minimum control speed) in a multi-engine airplane with one engine inoperative can lead to a loss of directional control. VMC is defined as the minimum speed at which the airplane can maintain controlled flight with one engine inoperative. Below this speed, the aerodynamic forces acting on the aircraft may cause it to yaw uncontrollably towards the inoperative engine due to the asymmetrical thrust produced by the functioning engine.

When this occurs, the aircraft may become difficult to control, and the pilot may not have sufficient rudder authority to counteract the yaw. This can lead to an adverse situation known as a "vortex ring state" or potentially result in a stall. Therefore, it is critical for pilots to remain above VMC when one engine is not functioning, as this is essential for maintaining controlled flight in a multi-engine aircraft.

The other scenarios—such as increasing altitude, improved climb performance, or stable flight—are not applicable at speeds below VMC, as they disregard the inherent risks associated with inadequate control authority in this situation.