"She is one"
Translation:Αυτή είναι μία
This is not a great question, because the sentence is incomplete, and therefore, a proper translation can not be made. The test question should be either "She is a woman" or "She is a girl". With the first phrase, the translation would be "Αυτή είναι μια γυναίκα". If the question was "She is a girl", the indefinite article changes from μία to ένα, making the translation "Αυτή είναι ένα κορίτσι. Γυναίκα is feminine, but κορίτσι is neutral.
Since Greek doesn't need a pronoun subject (as the ending of the verb indicates whether it refers to "I," "you," "he/she/it," "we" or "they"), your answer with "είναι" implies either he/she/it; and because you correctly used "μία," the feminine form, that implied you meant "she" perfectly.
Well, in this case, it just couldn't be "a" to begin with. It's right at the end of the sentence. ^.^ It depends on context. When you do want to give emphasis on the number of objects and that's where you want to focus on, you'd translate it as one. If not, it's just the indefinite article.
(In cases where it's unclear, we make sure to always add both translations.)