Final answer:
In Ableton Live, to make a track output in Mono, you should use the Utility plugin and set the stereo width to 0%.
Step-by-step explanation:
The plugin in Ableton Live that you use to make a track output in Mono is Utility. The Utility plugin can be used to adjust stereo width and it has a feature that enables you to set the width to 0%, which effectively sums the left and right channels together making the signal mono. This is useful for checking how a mix might sound on mono playback systems or to ensure compatibility across different listening environments.