"What is Péter waiting for?"
Translation:Péter mit vár?
There is no indication that the object (what) should be pluralized. Also, it is more likely that they would say "Mire var...", since one waits on something. Using the accusative 't' on 'what' as an object would more closely translate to "What is Peter expecting?" rather than "What is Peter waiting for?"
No, there is no indication that the object should be pluralised, and so both versions (singular or plural) should be accepted; similarly with -t versus -re.
If you entered a version which is correct but is not yet accepted, please report it as an additional acceptable translation and help improve the course while it is in the beta stage! Thank you :)
"Mikre" was in one of the multiple choice answers, which I didn't realize was also correct. It's mik (what in the plura) + re ("onto" ending). It's another way to express "what" as the object of an action (as opposed to "mit"). I don't know how many verbs allow this, so they might not always be interchangeable.