You give three translations: axis, axle and core. I use core and you say that that is wrong!. I think that the context of a sentence should decide the correct translation.
I've found this a lot with duolingo. With other sentences, you can argue because of the context, but in a sentence as short as this, it really should accept core as well.
It would be nice if could learn the 11,000 more common words first. I think Duolingo is trying to teach me about cars - so far I have had the words petrol, petroleum, axle, machine, mechanism.