The correct answer is that you don't know without more information about this non-linear device. Non-linear only rules out linear, but can otherwise mean anything else, including putting out a fixed signal regardless of the inputs.
To show that you understand the concept, you should explain that a linear device can produce at most only the frequencies put into it. It can change the amplitude and phase angle of the input frequencies, but it can't create new ones. It could, for example, attenuate either or both frequencies well past the noise floor, so even for a linear device, the answer is none, one, or both of the input frequencies.
What they were probably after with this question is for you to say that the non-linear device can produce additional frequencies beyond those you put in. If the non-linear device happens to include a product term, as Leon is apparently assuming, then you will get the sum and difference of the two frequencies included in the output.