The correct answer is 'To do something that causes a lot of new problems that you did not expect.' Key Points - The meaning of the underlined idiom/phrase 'Open a Pandora's box' is to do something that creates a lot of new problems that you did not expect. - 'Open a Pandora's box' - Example: Sadly, his reforms opened up a Pandora's box of domestic problems. - Therefore, the correctanswer is 'To do something that causes a lot of new problems that you did not expect.'