The D sixth chord is a 4-note chord consisting of the notes D, F#, A and B.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as D6.
Chords that are a superset of D6. The chords include more notes but always D, F#, A and B.
Chords that are a subset of D6. These contain less notes but all of them are included in D6.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the D6 chord. Feel free to save or share the image as needed.