"They sleep there."
Translation:Eles dormem lá.
Given that there is no context to this statement and that aí, ali and lá all mean "there" shouldn't all 3 options be accepted. We don't know if the bed or place where "they" sleep is near the speaker or listener or far away from both. Without more information how on earth is one suppossed to know that lá is the preferred option of duolingo. Frustrating.
This is why "aí" seems the most plausible answer, as the speaker is probably pointing to the area where they sleep. Otherwise, he would have said "over there", meaning far away. Reading the comments above, the consensus seems to be that all 3 forms of there should be accepted in this sentence, given that the context is unknown. Duo please review. Thank you.