Which is better for the blank? I think B is correct, but why is A not?
You ...................... it to her. Now, she will inform everyone about the secret party.
A) needn't have told B) shouldn't have told
You needn't have told her.
Why? Because she knows already?
If she knows already, and is a gossip, then she will inform everybody.( And might well have done so as soon as she found out.)
BUT - the "Now" implies that she will tell everybody because he told her something that she already knew (and didn't need to tell her anyway!) Why would that prompt her/cause her to tell about the party?
If she knew already, she has probably told everybody already, long before the conversation in this piece of dialogue!