1. is correct. In the active version, the noun phrase only what you need to say is the object of "say", so it must become the subject of the passive version, which it does in 1.
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1. is correct. In the active version, the noun phrase only what you need to say is the object of "say", so it must become the subject of the passive version, which it does in 1.
Note that the focusing adverb "only" is a modifier within the noun phrase.