"Your book was on the table."
Translation:तेरी किताब मेज़ पर थी।
One can use either teri or tumhari, if u are conversing in a formal manner u will say tumhari or aapki but if u r in an informal situation u would say teri and that is also optional, this is the shortcomings of the Duolingo or a computer that it can't correct either way।।।। And if u want to meet formality then learn sanskrit, u know pānini created sanskrit grammar and none of the languages in the world has correct grammar than of pānini's sanskrit grammar
Duolingo should probably use आप की/آپ کی (f.) or तुम्हारी/تمہاری (inf.) when teaching these phrases. तेरी/تیری is extremely informal and borderline offensive unless possibly among very close friends (Then again, I've never used it with mine). I should probably mention here that the most formal (आप/آپ) is always safe, no matter how well you know the other person.