Final answer:
Pushing the HDG/TRK selector on an aircraft's flight control unit selects the heading or track mode for navigation, not altitude, autopilot engagement, or cabin temperature adjustment.
Step-by-step explanation:
In the context of an aircraft's flight management system, pushing the HDG/TRK selector performs the following function: C) Selects the heading or track mode for navigation. The selector allows the pilot to switch between heading mode, which gives the aircraft a specific magnetic compass heading to maintain, and track mode, which directs the aircraft to follow a specific ground track regardless of wind conditions. This is a crucial aspect of the autopilot flight director system (AFDS), where the pilot can command the aircraft to change direction without manually steering.