Final answer:
The plain trim tool removes part of a selected clip without affecting others, shortening the clip by cutting from its start or end.
Step-by-step explanation:
When you trim using the plain trim tool in video editing software, the typical behavior is option A: The plain trim tool removes a portion of the selected audio or video clip without affecting adjacent clips. This tool allows you to shorten the length of a clip by cutting away unwanted parts from its start or end, depending on where you apply the trim. It does not split the clip into two (B), change the playback speed (C), or adjust the volume (D).