Ésta es mío.
The sentence "This is mine" is incorrect, since it is very confusing when pronouncing it, we could say that it does not make sense.
We change some words, so that the sentence is correct:
It's all we do.
The sentence is Spanish we have: "Esto es mío".
If the sentence were this way, the answer would be correct, but since it is not, it is incorrect.
