Koolkaren, you are right on. "Umano" is an adjective, and as such the feminine form is "umana". Since you are a girl your choice of "Io sono umana" is correct. The Italian version still has some bugs. Btw, if they wanted to say "I'm a human being" it would be "io sono un essere umano".
I would use both. Even better I would say "Io sono un essere umano", because in Italian "lei è umano" can also mean "you are somehow gentle", often in a humorous sense. But it's ages I don't use it with this meaning. I wonder if this nuance comes from Fantozzi's movies... http://www.youtube.com/watch?v=9q0EusYM0h4
I don't understand this sentence grammatically. To me it seems that the Italian sentence (Sono umano) is using the adjective ('humane') and not the noun ('human'). C/Should the answer be 'I am humane'?
There is another example in this exercise where an indefinite article is used to clarify the meaning. 'Sono un umano' I am human.
Yes, you are correct amalate. If your version wasn't accepted, you should click on the "report a problem" button. However, when we aren't 100% sure of ourselves we shouldn't do that, and later it's improbable to come back to the same issue. I often read the discussion comments and if they confirm my suspicions, I report the problem right then.
umanitario humanitarian, humane
Disclaimer: I'm not a native Italian, but I do have a dictionary. ;-p There may or may not be subtleties of usage which would allow umano as a translation of humane for all I know. In fact I suspect it can be used this way (sometimes), because (for example) reverso.net, as well as meanings for human, says:
essere o mostrarsi umano (con qn) to show humanity (towards sb), act humanely (towards sb)
It doesn't give any real world examples of umano meaning humane.
So if it is an acceptable translation (in some circumstances), and it has already been reported as an alternative translation (multiple times over the past two years) the course moderators must have other reasons for not accepting it as a translation (such as it is rare for it to mean that, or that the situation needs to be more specific, or there is a more appropriate word).