One variety of this veggie is the no.1 vegetable source of vitamin D, which researchers have begun to find may play a role in muscle building.
In this sentence, 'which' leads nonrestrictive relative clause, explaining 'vitamin D.'
I want to know if it's possible to use 'what' instead of 'which', and that clause is an appositive of 'vitamin D.'
In your sentence, the "which" clause is a nonessential relative clause. " and so is set off by a comma.
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In your sentence, the "which" clause is a nonessential relative clause. If you wanted to make it an appositive phrase, then it would look something like this: "One variety of this veggie is the number one vegetable source of vitamin D, a vitamin that researchers have begun to find may play a role in muscle-building."
Notice that this different construction begins not with a relative pron