fort - under French adjectives
The basic translation of the French adjective "fort" is "strong, full of energy, vigorous, robust." Secondary translations are "fat, corpulent"; "solid, resistant"; "difficult, hard to work"; etc. So, why would the translation of the adjective as "strong" be marked as incorrect?
It might depends of the sentence and the context. For exemple, in "Il a une taille forte", "forte" doesn't mean "strong". If you still think that in your sentence, strong should be good, send a feedback to duolingo