What should be ensured to avoid instability during flight?

Ensuring correct aircraft loading and balancing is crucial for achieving stability during flight. When an aircraft is loaded properly, the center of gravity (CG) falls within the specified limits set by the manufacturer. Proper balance means that the weight distribution is appropriate across the aircraft, preventing issues such as uncommanded yawing or pitching during maneuvers. An aircraft that is too nose-heavy or tail-heavy can be difficult to control, leading to potential safety hazards.

In contrast, other factors presented in the choices are not conducive to stability. For instance, flying at high altitudes during takeoff does not inherently relate to stability; in fact, it can introduce different aerodynamic challenges. Exceeding weight limits and ignoring fluid levels are both detrimental to the aircraft's performance and safety. Excess weight can compromise lifting ability and stall speed, while improper fluid levels can affect systems critical for stability and control. Therefore, maintaining correct loading and balance is the primary method to ensure stability during flight.