Which of the following is NOT included in the Essential Bus?

The Essential Bus in the Cessna 172 primarily powers the critical systems necessary for the safe operation of the aircraft during emergencies, ensuring that the pilot has access to essential flight information and communication capabilities in the event of a failure in the primary systems. Among the components typically included in the Essential Bus are the Air Data Computer (ADC), the Attitude and Heading Reference System (AHRS), and key communication equipment.

The transponder, while essential for air traffic control communications, does not fall under the category of critical systems that are considered necessary for the basic flight operation during an emergency. It is part of the non-essential systems, meaning that while valuable for situational awareness and compliance with air traffic regulations, it is not crucial for the aircraft's fundamental operations.

Comm 1 is a critical communication system, allowing the pilot to communicate with air traffic control and other aircraft, ensuring that safety and situational awareness are maintained. The inclusion of systems like the ADC and AHRS is vital for understanding airspeed, altitude, and aircraft attitude, which are fundamental for safe flight.

Therefore, the identified choice, the transponder, is not included in the Essential Bus since its function can be deferred in emergency situations where basic flight operations are prioritized.