"The fox returns to the park."
Translation:여우가 공원으로 돌아와요.
This sentence emphasizes the point made in the previous exercise about lack of consistency. Here "가" is accepted, though there is nothing that really differentiates the sentence type from "She goes up the mountain." Also, there is no reason to disqualify '에" for "으로" because the English sentence does not make clear that the fox is still on the way. The natural assumption is that the fox has arrived, so "에" would be quite appropriate.
'Returns" can mean either "to come back" or "to go back", depending on where the observer is.