What component fails if the BAT BUS fails?

The correct answer is the Electronic Horizontal Situation Indicator (EHSI) because the EHSI relies on the battery bus (BAT BUS) for its power supply. The BAT BUS is essential for providing electrical power to various components of the aircraft’s avionics system. If the BAT BUS fails, the EHSI would lose power and consequently cease to function, which is critical as it provides navigational and situational awareness information to the pilot.

Other options are associated with different power sources or circumstances. The attitude indicator, for instance, often operates on the standby bus or other independent power systems. Normal gear and flaps typically have their own hydraulic or electrical systems that do not directly depend on the BAT BUS. The GPS, while it can be influenced by various electrical systems, might still have power from alternate sources if the BAT BUS fails. Thus, the EHSI is the most directly impacted component when the BAT BUS fails due to its reliance on that specific power supply.