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Can you, please, help to choose the correct variant?

The person on the right in the picture might not/can't/must not be who you say it is.

The answer key says that it is must not, but I have never come across negative form of must to be used for expressing possibility, certainty or probability. Is it possible to use it like that and why are might not and can't are incorrect?

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    The answer key? What's the question? All three versions are perfectly valid, but by default they all mean different things. Specifically, might not is probably warning the addressee that it's possible he misidentified someone. Can't implies the speaker believes you've made a mistake, where must not would normally be an extremely emphatic alternative to can't (in which context speaker might well have just presented a conclusive argument / incontrovertible evidence that it's totally impossible for your identification to be correct).
    – FumbleFingers
    Mar 9 at 18:12
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    Along the lines of what @FumbleFingers said, please provide the preceding text or question (or state that there was none). It is impossible to determine what is intended without more context.
    – MarcInManhattan
    Mar 9 at 19:49
  • Thank you so much for your answers! I feel relieved to see the confirmation that all the three variants are quite valid. There is no any preceeding or subsequent context to this sentence, just this one sentence and that's it, as it's taken out of a grammar test for upper-intermediate students
    – dilek22
    Mar 11 at 15:21

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