C/Bb (C Major over Bb)
Notes, from the bass up
Bb (bass)CEG
A slash chord names two things at once. The part before the slash is the chord your right hand plays, and the part after it is the single bass note for your left hand. Read C/Bb as C Major with Bb in the bass.
The bass note here is not part of the chord itself. Your left hand plays Bb while your right hand plays the plain C Major shape above it.