What knob must be adjusted to change the rhythmic groove of the drummer?

Adjusting the swing knob is essential for changing the rhythmic feel of a drummer's groove. Swing adds a particular rhythmic nuance by altering the timing of the notes in a way that creates a laid-back or "swinging" feel, as opposed to a strictly even eighth-note rhythm. This knob allows you to essentially shift the placement of the notes, giving the music a broader and more dynamic expression, which is central to many styles of music such as jazz and funk.

Other options like the tempo knob control the speed of the overall playback rather than the rhythmic feel. The volume knob impacts the loudness of the track and doesn't affect the groove directly. The groove knob might imply adjustments related to timing or other characteristics, but the swing knob specifically targets adjustments to the rhythmic feel and placement of notes, making it the correct choice for altering the drummer's groove.