Can someone tell me why this wouldn't be "Que" instead of "Cual"? I'm generally confused as to when Cual is no longer supposed to be read as "which" but instead "what". Can someone clear this up for me?
I've found this pretty helpful: http://spanish.about.com/cs/grammar/a/que_vs_cual.htm
Depends on context, it could be actually referring to a theory, but calling a hypothesis a theory is a common mistake both in Spanish and English.
It isn't a mistake. It's simply that there's a specialized meaning within science for "theory," and then a meaning that is used outside that specialized context, and they're not the same.
All brontosauruses are thin at one end, much thicker in the middle, and thin again at the far end. That is my theory.
It's great that the only way Duo can understand me is if I yell. My Spanish is going to be far too aggressive..
Is there a bug in the program? I give the right answer, but it says it is wrong.
Without being able to see exactly what you put in, it's hard to tell. If there seems to be a bug, report it. (I have had this several times lately, and have put in either ¿Cuál es tu teoría? or "What is your theory?" and had it accepted every time.)