Select the most appropriate meaning of the underlined idiom in the given sentence.

A lot of work has gone on behind the scenes for the special event which is being planned next week.

Unknown to everyone

The most appropriate meaning of the underlined idiom in the given sentence is Unknown to everyone.

The idiom "behind the scenes" means that something is happening secretly or without the knowledge of the public. In the sentence, the work that has been done for the special event is happening behind the scenes, meaning that it is not being done in public view. The people who are working on the event are doing so unknown to everyone else.

The other options are not correct. To spread the news means to tell people about something. In the theatre means in a theater. To complete the arrangements means to finish making the plans for something.