This is the 352nd most frequent Turkish word.
mu
The Turkish word "mu" is a question particle used to form yes-no questions, equivalent to "is it" or "does it" in English.
In this sentence, 'mi' is used as a question particle asking for confirmation about the freshness of the apple.
Bu elma taze mi?
Is this apple fresh?
Here, 'mı' and 'mi' are used after 'Kitap' and 'dergi' to pose a question about the options being read.
Kitap mı, dergi mi okuyorsun?
Are you reading a book or a magazine?
In this case, 'mu' emphasizes the subject, questioning whether it was the person who said something.