The D major seventh chord is a 4-note chord consisting of the notes D, F#, A and C#.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Dmaj7.
Chords that are a superset of Dmaj7. The chords include more notes but always D, F#, A and C#.
Chords that are a subset of Dmaj7. These contain less notes but all of them are included in Dmaj7.
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 Dmaj7 chord. Feel free to save or share the image as needed.