In the US cinema released version there were actually three endings which the audience could vote for which they wanted to see.....the DVD (I have it on Import) has all these versions and if you go by the last ending there is no one answer as different people kill different characters as they are all being blackmailed by the various side characters who pop up through the film.
.....If your stuck on a quiz though try the butler or Tim Curry as the answer, it might work even though it's wrong.