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Your roommate just found a strange, alien-looking gadget in the back of the kitchen drawer and is holding it up in confusion.
Select ALL the grammatically correct questions they could ask to find out its purpose.
The correct answers are What is this bizarre gadget used for? and For what is this bizarre gadget used?
When asking about purpose, we use "What... for?". The most natural and common way to ask this in English is to leave the preposition "for" at the end of the question.
Moving the preposition to the front ("For what is this...") is grammatically correct but sounds quite formal or old-fashioned.
The option missing "for" is incorrect because "used" requires the preposition to indicate purpose. The final option is incorrect because it lacks the necessary subject-verb inversion for a question (it should be "is this", not "this is").