Why can't we use "của mình" instead of "của cô ấy?"
Because it doesn't specify anywhere else in the sentence who it is refering to
I wrote "Cái này là nhật ký của cô ấy" but it wasn't accepted. Was I incorrect?
As far as I know, này can only be used in conjunction with a head noun, not stand alone.
The multiple choice version of this drops the 'quyển'--why? I'm assuming she doesn't have plural, generic, diaries.
Đây is already singular.
If we wanted to say “that is her diary” what word do we use for “that”?