smc

The given statement tells us that the weather being not fair is the only thing that can cancel the picnic on Sunday. It does not say that the weather being not fair necessarily cancels the picnic, so choice (A) is out. If the weather is fair, however, nothing can stop the picnic from happening, so choice $\boxed{\text{If it is fair Sunday, the picnic must be held.}}$ is correct. Choice (B) is incorrect, because if the picnic is cancelled, then the weather must be unfair, because the weather is the only thing that can stop the picnic. Choice (C) is incorrect, because it is the contrapositive of choice (A), so (A) and (C) are logically equivalent. Choice (D) is incorrect, because the picnic must be held if foul weather is not there to prevent it.