Yeah, it sucks - 1000 times reported that it is used AT LEAST as often as άντρας and still it's always a gamble if it is accepted or not.
If the exercise is a listening one, then it definitely won't be accepted. There are no alternatives for listening exercises, the translation has to match the original sentence exactly.
Otherwise, it should be accepted. It's in the alternatives.
I'm confused! I answered wrong and it said the correct translation was "this is a man". Does "Αυτός είναι ήδη άντρας" mean 'this is already a man' as well as 'he is already a man'? "This is already a man" doesn't really make any sense...but clarification would be helpful!