The negative result of poor longitudinal stability in an airplane is?

The correct answer highlights that poor longitudinal stability in an airplane primarily manifests as a difficulty in controlling pitch. Longitudinal stability relates to how well an aircraft maintains its pitch attitude during flight. When an aircraft has poor longitudinal stability, it tends to oscillate in pitch or may not return to the desired angle of attack after a disturbance. This makes it challenging for pilots to maintain stable flight, requiring continuous input and correction, thus impacting overall control and safety.

While the other answers may have some relevance in certain contexts, they do not directly address the main impact of poor longitudinal stability. For instance, a tendency to nose dive excessively can be a result of control issues, but it doesn't encapsulate the broader difficulty in maintaining control of pitch. Similarly, increased fuel consumption and reduced lift efficiency may be effects of various factors but are not exclusive indicators of poor longitudinal stability. This reinforces why the focus on pitch control is critical when discussing stability issues in aircraft.