Which instrument is included in the essential bus of the Cessna 172?

The Primary Flight Display (PFD) is included in the essential bus of the Cessna 172 because it provides critical flight information to the pilot. The essential bus is designed to maintain the operation of vital systems in the event of an electrical failure, ensuring that the pilot has access to key instruments necessary for safe flight and navigation. The PFD consolidates important data such as attitude, airspeed, altimeter, and navigation information, all of which are crucial for maintaining situational awareness and safe operation of the aircraft.

In contrast, other options like strobe lights and landing lights, while important for visibility and safety, are not critical for the immediate operation of the aircraft and would not be prioritized in the event of an electrical failure. The beacon serves a similar purpose in terms of visibility but does not provide essential flight data. Thus, the correct choice emphasizes the importance of maintaining access to essential flight information during critical flight operations.