The German word for the interrogative adjective "which?" is "welcher?". It declines in exactly the same way as the definite article "der". The declination of the interrogative adjective for the cases which we have met so far are as follows:
Here are a few sample sentences showing the use of the interrogative adjective.
Welcher U-Bahnhof ist das? ("Which underground station is that?")
Welche Linie geht dahin? ("Which line goes there?")
Welchen Zug nehmen Sie? ("Which train are you taking?")
Von welchem Gleis? ("From which platform?")
Mit welchem Bus fährst du? ("With which bus are you travelling?")