The most natural way to say it would be simply without any pronoun: Я ще не зробила домашнє завдання.
The second most-natural would be Я ще не зробила своє домашнє завдання. That sounds strange though, as if you were emphasizing that you haven't done your homework yet, and you're supposed to also do homework for someone else :D
The third place is Я ще не зробила моє... Usually repeating the person in the possessive pronoun that corresponds to the subject is as awkward as double negative in English. However, in this course it's allowed.
So, all of these are accepted.