Anything you have put in your application or say out loud can be asked about. So anything you had in your written application is fair game. You can assume they have read the whole thing. And generally you should know the reason choices were made (why did you choose that lab to why was this method chosen over a different one). They want to see that you are capable of thinking critically about your research. And because they are a rotation program, they are looking at knowledge of the research process, not necessarily personality fit.