"The man loves you."
Translation:Mannen elsker deg.
du = singular subject pronoun
deg = singular object pronoun
dere = plural subject and object pronoun
In the above sentence, 'you' is the object. Since English uses 'you' for both the singular and the plural, and there is no clear context for this particular sentence, you have the choice of translating it with either 'deg' (singular) or 'dere' (plural).
"Mannen elsker deg." is our default translation, so if that was marked as incorrect there must have been a glitch.
If this was a multiple choice task, you need to pick both "Mannen elsker deg." -and- "Mannen elsker dere." for it to count as correct.