"뭐 해?" (what Mashup answered above) is fine, but if the person is older than you, then you're gonna want to put a 요 on the end...뭐 해요?
The second one written there (당신은 무엇을 하고 있습니까?) is, in my opinion, way to literal and formal.
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당신은, 무엇을 하고 있습니까?
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besides that good answers above, for Present Continuous Tense specifically you can say => 뭐 하고 있어요? [polite spoken form]