As far as I've understood, the hints are shared - they all show up when the word they're linked to is used. Therefore, they won't fit every sentence. Sometimes "behind" is the better translation, other times "in the back of" is right. It would be an enormous job to manually type in translations to every word, every phrase, every sentence..! However, your safest bet is to choose the first word in the list:)
The hint says "in the back" and not "in the back of" which is not the same thing. In English, you can say "He is behind the girl." or "He is in the back." Oddly, you can say "He is in the back of the classroom." giving a location after "of" but not a person or a thing. People do often say "class" for "classroom" though. The other way to say that someone or something is behind someone else or something else would omit the definite article: "He is in back of the girl." or "The glass is in back of the plate." but that form is rarely used. I might however say "The box I want is in back of the others." Somehow, it seems to emphasize how hard it is going to be to get it. I would tend to use "behind" if it were behind only one box.
Of is a preposition and we do not use in the back of in english unless you specify a prepositional phrase as such the comment I'm replying to states.
"He is standing in the back of the classroom"
In the back is a prepositional phrase is self because in is a preposition just like of.
It may just be my dialect, but I use in back of and behind pretty interchangeably, the materials I use to teach ESL use them pretty interchangeably, and I'm not sure why there's a distinction being made here. He's in back of the girl and he's behind the girl, the plate is behind the glass or the plate is in back of the glass - mean the same. Now, I'd object to in THE back of or AT THE back of - because that implies that the thing is inside whatever it is, but you can't use behind in that circumstance, either.