"What are you laughing at?"
There's no reason why this sentence should not accept "你" in lieu of "你們".
I also was marked wrong for using 你 instead of 你们.
This is the third question in a row that I got wrong for an answer Duo should have accepted. Good lord, this is a frustrating course.
It gets more frustrating as the months wear on, with improvements seemingly having ground to a halt.