a) Skim through the dialogue and find a word beginning with un-. What does it mean? When do we use this prefix?

The word is 'unbirthday'. It means a day that is not your birthday. The prefix 'un-' is used to create an opposite meaning or to indicate negation. In this context, it's used to refer to any day that is not a birthday, highlighting the humorous concept of receiving presents on days other than one's actual birthday.