How can water enter an aircraft fuel system?

Water can enter an aircraft fuel system primarily through precipitation, condensation, or faulty seals, making this the correct choice. Precipitation refers to rain or snow that can potentially get into the fuel tanks during refueling or when the aircraft is sitting on the ground. Condensation occurs when water vapor in the air cools and forms droplets, especially in tanks where temperature fluctuations occur; this can lead to water collecting in the fuel system if not properly managed. Faulty seals can fail to keep moisture out, allowing water to seep into the fuel storage areas.

The other options do not accurately represent the common ways water can intrude into the fuel system. For example, navigation systems are not related to the introduction of water; their malfunction has no direct effect on moisture entering fuel storage. Likewise, while engine leaks may lead to other issues, they do not specifically contribute to water entering the fuel system. Fuel overpressure can cause leaks or spills, but it is not a means of water intrusion. Therefore, precipitation, condensation, and faulty seals cover the most prevalent methods by which water can compromise an aircraft's fuel system.