If I were asked the question my thought/answer process would be something like:
Firstly I would say: "I don't know"... nothing wrong with honesty.... I'm not an ATCO yet... "I would hope to have learnt that during my training"... "Things I would consider are: wake, fuel, passengers, priority"... shows you are not bluffing, but have knowledge of things that may affect the decision...
There is nothing wrong with not knowing the answer to a question or two... When I had my interview, I thought I was awful with a question about runway usage, and asked if I would change my decision based on who was on board (the Queen etc..), whether a plane was allowed to take off given a certain set of circumstances.. I said I wasn't sure, and would hope that by the time I had to make that decision I would know, but If I didn't the first thing I would do would be ask my supervisor what the law/procedure was... I then went on to say what things I believe would come into the equation...
James.