Try "who's are these pants" that doesn't work either. Still a long way to go with this course!
who's is not the same was whose. Actually there is no such word as who's so I'm not surprised it would not accept it!
Yes, you are right Judit, my typo, but I tried this on a Tablet, given a choice of words to rearrange including "whose" "whose pants are these" works, while "whose are these pants" isn't accepted. "trousers" is also still not accepted.