The question in the lesson uses the word deep. The correct answer was "Profondeur"?

These are not specific, literal translations. Basically in English we can say 'x is 5 metres deep' or 'x is (or has) 5 metres (in/of) depth' and in French «(faire) 5 metres de profondeur» or «(être) profond/e/s de 5 metres», without any significant difference in meaning. If it was a single choice quiz, only 1 of the correct answers is presented, which doesn't mean the other is wrong. On multichoice, usually the 2 correct options would be presented. 

There is nothing in the lesson that indicates context is important in deciding between the options presented. If context is important, expansion either in the lesson or here in the forum would be useful.