"Which objects are in the room?"
Translation:Welke voorwerpen zijn er in de kamer?
It is subtle, so well done flagging this. But, "welke" demands that you identify the objects (stoel, tafel, schilderij, ficus), "wat voor" demands that you categorise them (furniture, decoration, plants).
Compare: Q: Welke film wil je zien? A: The Avengers End Game with Q: Wat voor film wil je zien? A: A thriller.
You can use staan if objects that can (potentially) fall over are involved (or since it's a question, you can use it if you think that that kind of objects are in the room). Zitten doesn't work here. There is an extensive explanation on zijn/staan/liggen/zitten (and that only covers the basics of when to use which…sorry), you can find it here: https://www.duolingo.com/comment/5785064
It probably is linked to the size of the thing (in this case a room) the objects are in. Zitten normally is used for things inside containers, bags, baskets, boxes, pockets, etc. I can't think of anything a person normally goes into (like a room, a building, a forest) for which zitten is used for the objects inside it.