Which of the following is NOT a problem associated with structural icing?

The reason "Improved fuel efficiency" is the correct choice as the option that is not a problem associated with structural icing is that structural icing typically leads to negative impacts on the aircraft's performance, rather than any improvements.

When aircraft accumulate ice on their surfaces, the aerodynamic smoothness is compromised, leading to increased drag and weight. These factors directly contribute to decreased performance metrics such as climb capability and handling. Additionally, increased landing speeds are often a consequence of the added weight and altered aerodynamic characteristics caused by the ice.

In contrast, improved fuel efficiency would imply that the aircraft is operating in a more aerodynamic state, which is not the case with structural icing. Thus, this option stands out as it does not align with the detrimental impacts of icing on overall aircraft performance.