The B dominant flat ninth chord is a 5-note chord consisting of the notes B, D#, F#, A and C.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as B7b9.
Chords that are a subset of B7b9. These contain less notes but all of them are included in B7b9.
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 B7b9 chord. Feel free to save or share the image as needed.