Many geographers consider what to be one of the boundaries that separate Europe and Asia?

There is no hard and fast answer to this and many geographers do not consider Europe and Asia to be separate continents as there is no logical physical divide between them. However, the most likely answers would be 'Ural mountains', 'Caucasus mountains' and the 'Bosphorus strait'. As the question asks for 'one of the boundaries', any of these should suffice.