"You are a man."
Translation:Sei un uomo.
"Sei un uomo" is the informal way to say "you are a man". "Lei è un uomo" (with a capital L for Lei) is the formal/polite way to say "you are a man".
In theory, "Lei è un uomo" could also mean "She is a man" but in most cases the speaker is unlikely to mean this, so you have to use the context to figure out what they mean. :)
Duolingo just gets worse! I admit what I chose as my answer may be wrong. However, to complete the answer it was one of these new answers where you move a box with a word to create a series of boxes in a line that is the answer. There was no box with "Lei" and no box with a 'e' with a grave accent. Therefore, there is no way I could have given the answer, "Lei 'e grave' un uomo".
The way I was taught: Lei è = You are (only used formally, or with older acquaintances, shows respect) Tu sei = You are (regular use) So if you met your friend for coffee, you could say "Tu sei una ragazza," to her, meaning "You are a girl." However, if you met your friend's grandmother for coffee, whom you had never met before, you could say, "Lei è una donna," which looks like "She is a woman," but you are implying "You are a woman."
Apostrophe (') usually means that something is left out: ridin>ridin', he is>he's, cannot>can't etc, similarly in other languages. In "un uomo" there's nothing that can be left out, so there's no reason to use the apostrophe, unlike l'uomo, which could be explained as contraction from (incorrect/historical?) "lo uomo".
Now I read this occurs also in Italian, but in normal language it's NOT like in French. The polite form in Italian to address somebody is 3rd person singular (e.g. Lei è = vous êtes), though you must be careful because "lei" also means "she" (lower case in this case), or the pronoun may be omitted and then it can mean il/elle/vous. If this is too concise, here's some more about it: