D 6/C#
The D 6 chord consists of the D (Perfect unison), F# (Major third), A (Perfect fifth) and B (Major sixth) notes, with C# as the bass note.

How to play the D 6/C# chord
- Play C# as the lowest bass note in the left hand
- Find D (Root) on the piano keyboard
- Add F# (Major third) — 4 semitones above D
- Add A (Perfect fifth) — 7 semitones above D
- Add B (Major sixth) — 9 semitones above D
- Press all 5 notes simultaneously to sound the D 6/C# chord