It's a trick question.
There isn't a "proper" place for a "bone plate" because in formal dining, there is no such thing as a bone plate, as one does not deliberately put bones in one's mouth. If you get a bone in your mouth while eating a piece of fish, you are supposed to discreetly remove the bone from your mouth, then place it on the edge of your dinner plate. With any other kind of meat or fowl, you should be cutting around the bones with your knife and fork and leaving them on the dinner plate where they belong, UNLESS you are eating marrow, in which case you may, if necessary, hold the bone down with your fingertips, whilst using the marrow spoon in your other hand to remove the marrow. Then either eat it from the spoon, or spread it on the provided toast points, which may, of course, be eaten with the fingers. (Of course, it hasn't been common to serve marrow that way in formal settings for nearly 200 years, but who pays attention to that kind of trivia when making up a trick question?)
In a setting that is informal enough that people are deliberately putting bones in their mouths, no one at all is going to care where the discards vessel is, or whether it is a plate, a bowl, or a tin bucket.