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Who is she?
Literal
She [topic-は] who is [question-か]?
A textbook identification question — the polite copula です linking the topic 彼女は to the question word 誰, with か marking it as a question. The structure is the same shape as Xはなんですか ('What is X?') or Xはどこですか ('Where is X?'). For this particular question English would likely use 'Who is that?' if pointing at a stranger; 彼女 in such a context is closer to 'that woman' than the more romantic 'she / girlfriend.'