The DC EMER BUS powers the ________.

The DC EMER BUS is an essential component in the electrical system of the aircraft, specifically designed to supply power in emergency situations. It provides direct power to critical systems that must remain operational during failure scenarios, which includes both flight instruments and the engine and APU fire extinguishers.

Powering the flight instruments ensures that the pilots can continue to receive vital information related to the aircraft's operation and its surroundings, which is crucial for maintaining control in emergencies. Additionally, the ability to activate the fire extinguishers for both the engines and the Auxiliary Power Unit (APU) is critical for safety, as it allows the crew to respond effectively to potential fire hazards.

This dual functionality is what makes the selection correct, as the DC EMER BUS serves to power both the flight instruments and the fire extinguishing systems, ensuring that all necessary safety and operational functions are maintained during emergency conditions.