Why not "it is quite possible"? That means that you are cautiously (and/or politely) expressing a possibility. Duolingo gives "completely possible" as the answer, but you don't say that in English. It doesn't mean anything anyway. If something is possible it is possible, 100% possible is meaningless. If you mean certainty then you say certainty.
Could anyone explain how 'completamente' and 'assolutamente' are different? English is not my mothertongue unfortunately and in my native language translations of both words are the same...So I am a bit confused when should you use 'completamente' and when 'assolutamente', as I guess, they are not interchangeable ?
I can try to help, i'm really not sure of all the correct grammatical terms, i more just have a feel for when to use a certain word over another, as most native speakers do with their mother tongue.
Absolutely is used with ability/action, completely is used with quantity/time. For example, If someone were to ask you regarding a project/report, "Can you have this to me by the end of the day," or "are you able to do this" you could say absolutely (your ability to do this thing is certain, whole, absolute), but you could not say completely - it wouldn't make sense in that context. Regarding completely, If someone were to ask you "How high is the pool filled?" you could say it is completely filled but not absolutely (absolutely filled can be used, yes, but far more people are likely to use completely)
If that doesnt help think of it like this: A collection of bottles or cans can be complete. Meaning you have collected them all, there is no more - your collection is finished, it is complete. And your ability to collect bottles and cans is absolute. Meaning it is perfect, it is certain (as evidenced by you collecting all of them). Like a math problem, 2 + 2 = 4. This equation, and math in general, is absolute, it is true, factual, cannot be changed, it is what it is; it is not complete, it is absolute.