The hint shows that nessa can mean in or on . It also says that it can mean this or that. How do I know whether to use this or that? Couldn't I translate as 'on that farm' which would make more sense in english but which was marked as incorrect?
The word "nessa" is composed of the preposition "em" (which usually means "in", "on", "at", it doesn't have an exact equivalent) and the pronoun "essa" ("that" in European Portuguese, "this" or "that" in Brazillian Portuguese). So without context, there are a lot of possible translations as you can see.
So if you find a similar example, and your answer isn't accepted, don't forget to report it. =)