C is the correct answer because the author used “would be” to represent his/her opinion
The correct answer is the first one.
He describes Mrs. Miller's behavior as "unprecedented." By this, he is saying that, of all the parents he has ever known, her behavior is unusual, singular, or unique.
Quite simply, it is strange that she would allow her daughter to run around town with a strange man. Mrs. Miller's behavior is more than silly -- it is downright unheard of.