Which of the following must pilots include when reporting icing conditions?

When pilots report icing conditions, it's essential to include the type of aircraft they are flying. This information is crucial because different aircraft handle and respond to icing differently based on their design, size, and capabilities. For instance, larger aircraft may be equipped with advanced de-icing systems that smaller aircraft do not possess. Therefore, knowing the type of aircraft can help air traffic control and other pilots gauge the severity of the icing conditions experienced and inform operational decisions that could prevent incidents or accidents.

The other options, while they may provide additional context, do not hold the same level of importance in the specific context of reporting icing conditions. Flight duration, weather forecast, and type of fuel used do not directly relate to the aircraft's susceptibility to icing and its operational impact.