The D diminished seventh chord is a 4-note chord consisting of the notes D, F, Ab and Cb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Ddim7.
Chords that are a subset of Ddim7. These contain less notes but all of them are included in Ddim7.
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 Ddim7 chord. Feel free to save or share the image as needed.