What is the result of cold soaked fuel on the underside of the wing?

The formation of thin frost on the underside of the wing occurs when cold soaked fuel, which is fuel that is significantly colder than the surrounding air temperature, comes into contact with the wing's surface. This phenomenon is due to the temperature differential that causes moisture in the air to freeze upon contact with the cold surface of the wing. Thin frost can negatively affect the aerodynamics of the wing by altering its shape and reducing lift, which is why it is critical for pilots and ground crews to be aware of fuel temperatures and their potential impact on aircraft performance.

While other options mention ice development, weight concerns, and fuel efficiency, they don't accurately describe the immediate consequence of cold soaked fuel on the wing's surface in this context. The emphasis on thin frost illustrates a direct and relevant effect that can compromise flight safety and operational performance, making it the most suitable answer.