How do I play this part on my french horn? From Jacques Ibert - Trois pièces brèves
Since you haven't circled the other instances of the tremolo, I'm guessing you're looking for a good fingering combination, not just an explanation of the notation.
I would play this on the B-flat horn using fingering 2-3 for the C-sharps and just 3 for the D-naturals. You could also try the F horn with 1-2 for the C-Sharp and 1 for the D, but that might be a little squirrelly on the F horn.