Answer:
16. Weißt du, wann der Zug abfährt? (last option)
17. Er möchte wissen, ob wir morgen zu Hause essen. (second option)
18. Sag mir, wo die Blumen jetzt alle sind. (first option)
Explanation:
16. In a dependent clause, the separable prefix stays with the verb, which is placed at the end of the sentence.
17. In a dependent clause, the verb is usually placed at the end of the sentence.
18. The subject should be placed after the subordinating conjunction and the verb, at the end of the sentence.
