Aristophanes is considered one of the greatest Greek writers of comic drama.
Mortified in this context would mean: To cause embarrassment.
Janis was "embarrassed" to learn that her brother was reading her diary.
The other answers are wrong because: You would not be "made enjoyable" because your brother read your diary. You would not "Be discovered" because your brother read your diary. You would not "be happy" because your brother read your diary.
Hope that helps!
-Chris