"Whose cups are these?"
Translation:Wessen Tassen sind das?
"Dies" here is in the nominative and isn't declined. If you reversed it, you'd say "Dies sind ihre Tassen."
You would need to say something like "Sie haben diese Tassen" which takes a plural accusative ending. It's acting as an adjective modifying Tassen rather than the subject.
It's because in "Wessen Tassen sind das" ("whose cups are these"), "das" is acting as a pronoun, not an article.
"Das" can be both a pronoun (substituting a noun, like "that" in "That is good" = "Das ist gut"), and an article (accompanying a noun, like "the" in "The food is good" = "Das Essen ist gut"). "Die", on the other hand, can only be an article ("the") like in "The newspaper is good" = "Die Zeitung ist gut".
You couldn't say "Wessen Tassen sind die" the same way you can't say "Whose cups are the" in English.
A previous question in this practice lesson had the combination "dieser Tassen" indicating that the sentence was in the Genitive case. So, on this question, I was certain it was "dieser" (Genitive) again......but obviously that is incorrect. Suggestion: it would be VERY helpful if the answer provided a one or two word explanation like "Genitive case" or whatever. I think that would decrease the number of questions asked when people have a wrong answer! :-)