D major chords for piano with alternative bass notes presented by keyboard diagrams.
Explanation: The slash in the chord name indicates that the second letter is added as a new bass note. D/C# should therefore be read as D Major chord with a C sharp note in the bass and D/C should therefore be read as D Major chord with a C note in the bass and so forth.
Theory: D/C# is identical with the third inversion of Dmaj7, D/C is identical with the third inversion of D7 and D/B is identical with the third inversion of D6.