Final answer:
The gyroscope is the primary source of attitude readings for the Basic Flight Instrumentation briefing guide, providing critical orientation information for the aircraft.
Step-by-step explanation:
The source of attitude readings for the BFI (Basic Flight Instrumentation) briefing guide is the gyroscope. The gyroscope is used to measure and maintain the orientation and angular velocity of the aircraft, which is essential for the attitude indicators that tell the pilot whether the aircraft is pitching, rolling, or yawing. The other instruments listed: accelerometer is used for measuring acceleration and may contribute to other cockpit readings like G-forces, and magnetometer acts as a compass to measure directional orientation relative to the Earth's magnetic field. These sensors combined can provide comprehensive data for various flight instruments, but specifically for attitude readings, the gyroscope is the key component.