What factor can affect the density of fuel in the tank?

The density of fuel in the tank is primarily affected by temperature. As the temperature of a substance increases, its molecules tend to move further apart, leading to a decrease in density. Conversely, if the temperature decreases, the molecules contract and the density increases. This change in density due to temperature variations can be significant in aviation fuel systems, as fuel density directly impacts the fuel's performance characteristics, including how much fuel can be stored in a given volume and the overall weight of the aircraft.

The other factors listed, such as humidity level, altitude, and air pressure, do not have as direct an influence on the density of fuel within the tank. Humidity primarily affects the amount of moisture in the air rather than the fuel itself. While altitude and air pressure do have some effect on fuel properties, they are secondary compared to the impact of temperature changes on the density of the fuel in the tank.