The D half-diminished chord is a 4-note chord consisting of the notes D, F, Ab and C.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Dm7b5.
Chords that are a subset of Dm7b5. These contain less notes but all of them are included in Dm7b5.
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 Dm7b5 chord. Feel free to save or share the image as needed.