What should each fuel quantity indicator read during level flight with fuel equal to unusable fuel supply?

In aviation, each fuel quantity indicator is designed to reflect the amount of usable fuel in the tank. When the fuel level is equal to the unusable fuel supply, it implies that the operational fuel supply has been fully depleted, and the aircraft is now left with fuel that cannot be used for engine operation. This is typically measured from the lowest acceptable level of usable fuel in the tank, which is defined by the manufacturers of the aircraft.

During level flight, if the fuel quantity indicator shows zero, it indicates that there is no usable fuel remaining for engine use, aligning with the scenario where the fuel quantity equals the unusable fuel supply. This reading is critical to pilots, as it alerts them to the risk of running out of fuel. Thus, the correct indicator reading in this situation would indeed show zero.