Hint!
A top tip that works for so many puzzles: draw a sketch.
Remember that every box is labelled wrong. Consider each box and think what you could deduce from seeing one fruit from it.
Solution
You need to look inside the box labelled 'apples and oranges'.
Say you pull out an apple: you know that box contains only apples. It can't contain both because the label was wrong.
Now you are left with the box containing oranges and the one that has both. The one labelled 'oranges' does not contain oranges, so it must contain both.
That just leaves the last box, which must be 'apples and oranges'
Another clue is that the question suggests you can work it out by looking inside one box. Logically, there is no difference between the box labelled 'apples' and the one labelled 'oranges' - the odd one out is the one labelled 'apples and oranges', so go for that one!