What could be the reason to mark आपकी wrong?
It shouldn't have been marked incorrect, as "आपकी" is just a more formal way of saying "तुम्हारी". "उसने आपकी चिट्ठी पढ़ी।" should be correct too.
In hindi, we can say aapki,tumhari,or teri, many times,we use teri for friend ...if we want to give respect to someone we use aapki
Yeah
Is "उसने तुम्हारी चिट्ठी को पढ़ा" OK?