"Which objects are in the room?"
Translation:Welke voorwerpen zijn er in de kamer?
I wrote "Welke dingen zijn in de kamer" and it was marked as incorrect. I'm not sure why.
Also if you leave out er it sounds unnatural and a bit awkward. I'm not sure why, if you change zijn to liggen (Welke voorwerpen liggen (er) in de kamer?) you can leave out er without making it unnatural.
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.