Usable fuel is included in which aircraft weight measurement?

Usable fuel is included in the useful load measurement of an aircraft. Useful load refers to the weight of all useful cargo, which encompasses passengers, baggage, and fuel that can be used during flight. This is a critical metric for determining the aircraft's capacity to carry loads safely and efficiently.

In contrast, empty weight represents the weight of the aircraft without any usable fuel, passengers, or cargo. It includes only the airframe, engines, and any other components necessary for the aircraft to operate, but not any load that can be utilized in flight.

Flight weight, on the other hand, typically refers to the total weight of the aircraft at the moment of takeoff, which includes empty weight and usable fuel along with any payload. However, since the question specifically asks about what constitutes usable fuel in relation to the options given, useful load is the most accurate choice.