Ok setting aside GMAT..and its rules...following will be an usage note on 'which' as a relative pronoun...
The relative pronoun which
is sometimes used to refer to an entire sentence or clause,
rather than a noun or noun phrase,
She ignored him, which proved to be unwise.
They swept the council elections, which could never have happened under the old rules.
While these examples are unexceptionable, using which in this way sometimes produces an ambiguous sentence.
In otherwords which can refer to anything that is noun or noun phrase, a sentence or noun clause - especially as an object of the verb or preposition.
Now, coming back to GMAT, I am not sure if the same applies... looking at this SC it seems it does..