Answer and explanation

Correct answer: D. The staff meeting, which was scheduled for Monday, has been moved to Tuesday.

The relative clause needs a verb: 'which was scheduled'.
Answer: The staff meeting, which was scheduled for Monday, has been moved to Tuesday.
Why not the others: The staff meeting which was scheduled for Monday has been moved to Tuesday, this: it is incomplete. The staff meeting, which scheduled for Monday, has been moved to Tuesday.: the relative clause lacks 'was'. The staff meeting, what was scheduled for Monday, has been moved to Tuesday.: 'what' is not a relative pronoun here.

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