The possessive adjectives in Italian are
first person singular: mio/mia/miei/mie
second personal singular: tuo/tua/tuoi/tue
third person singular: suo/sua/suoi/sue
first person plural: nostro/nostra/nostri/nostre
second person plural: vostro/vostra/vostri/vostre
third person plural: loro
The list is in the usual order, with the full declension for the masculine and feminine form, singular and plural. Note that the plural third person form is invariable.
These adjectives are also used as pronouns, so they correspond to the English forms
first person singular: my/mine
second person singular: your/yours (archaic: thy/thine)
third person singular: his/her/hers/its
first person plural: our/ours
second person plural: your/yours
third person plural: their/theirs
The difference between Italian and English is that in Italian the adjective agrees with the gender of the possessed thing, whilst in English it agrees with the gender of the possessor.
Since the English pronoun is in the third person singular, the same should be used in Italian:
La camicia rosa è sua
The same sentence would translate the English The pink shirt is his.
The suggested translation by Duolingo is disputable: in this case the article in front of the possessive is not at all required: it can be used for special emphasis in standard Italian; however, some varieties of Italian use it regularly and it may sound strange to people like me.
By the way, la camicia rosa e la tua means a very different thing from la camicia rosa è (la) tua. The latter is the pink shirt is yours, the former means the pink shirt and yours. Never forget the accent on è (present tense, third person singular of essere).