How is RWY TRK armed for takeoff?

To arm RWY TRK for takeoff on an A320, selecting a heading on the Flight Control Unit (FCU) is the correct procedure. This action sets the desired takeoff track that the aircraft will follow once airborne, utilizing the autopilot’s lateral navigation capabilities. By entering the specific heading that corresponds to the runway orientation, the flight management system can accurately manage the aircraft's trajectory in relation to the ground track.

The other options don't achieve the desired outcome for arming RWY TRK. Selecting a runway in the flight plan provides important data regarding the flight’s route and performance calculations but does not directly activate the RWY TRK functionality. Pushing the heading knob during takeoff may be considered for adjusting the aircraft's heading, but it does not specifically relate to arming RWY TRK. Lastly, pulling the heading knob is typically used for disengagement of the current heading and does not initiate the RWY TRK function. This makes selecting a heading on the FCU the essential step needed for successfully arming RWY TRK during takeoff.