"She is a girl."
Translation:Ea est puella.
It is flexible because the endings/noun inflections/verb conjugations convey the meaning. Still Wheelock's Latin textbook emphasizes that Romans often used a Subject-object/adverb/etc.-verb order. The verb often came last as sort of a dramatic unveiling at the end because one doesn't know what the message is often until the verb is mentioned.
If that is true, then they will be adding some acceptable translations soon. For now, I know that (for this exercise) you have to get the verb in the middle. I.e. Ea est puella or Puella est ea. Lol, when I came around to it the second time, all I could remember was that I had gotten it wrong the first time. ;) Now I remember - verb in the middle. Easy.
To suggest word order doesn't matter is incorrect. It can and does change shades of meaning, specifically emphasis.
There is a standard word order (SOV--or, often, OV with the subject implied in the verb) that should be the basis of unemphatic sentences here.
That's not to say the others should be marked wrong, but there should be a suggestion for a better answer.
I learned classical latin during my scolarity. We would generally place the verb at the end of sentence, or, at the end of a significant segment of the sentence. Not always the last word but it's the general way. As tzznandrew wrote it, there is a standard word order and subverting it is indeed meaningful, so you definitely can't say word order doesn't matter. Sure, you can learn it without knowing the grammatical structure, but it's clearly not the best way, always better to grasp the logic of the language.