Translation:Which buses are the French tourists sitting on?
I think melyik/melyek works like adjectives do. We would say "a piros buszokon" . Piros (red) stays in the singular form here.
So, use melyik if it is directly before the noun, even if it is plural.
However, you can use "melyek" as a standalone:
-Három kedvenc nyelvem van.
-Tényleg? Melyek azok?
-I have three favourite languages.
-Really? Which ones are those?