It is understandable, and as long as you use the right intonation it would even work in speech, but it is not very elegant if not wrong. The normal way to form a question like this in English is to use a version of the auxiliary verb "do" as in Duolingo's model answer. I guess Duolingo wants you to follow the norm. I believe "declarative questions" like the one you suggested are treated as second class citizens in the grammatical world.
Concordei. Quando encontrei esta frase hoje (2018-11-28), ouvi a voz feminina dizendo «següiu». Esta pronunciamento da palavra é usada numa região do Brasil? Repito esta pronunciamento, mas estou curioso se esta é comum num lugar no mundo lusitano. P.S. Esta mulher tem uma problema comum com a letra «u», há muitas vezes quando encontrava uma frase com a palavra «quem» e ela disse «qüem».
That's a question for Duolingo.
This problem of ambiguity comes up all the time and it seems to me that once you know about it you can make life easier for yourself by using favoured patterns. For example, here "your" seems best, but in a question like "Ele seguiu o seu irmão?", "his" is probably the best bet.
If you know your answer is correct, though, whatever it is, ask for it to be accepted and eventually all the options will be covered.
Both are correct.
You asked a similar question here: https://www.duolingo.com/comment/13648079 and the explanation is the same, but perhaps Ruama's comments in this discussion will also help with some other situations: https://www.duolingo.com/comment/8843937
By the way, Ruama's opinion is that it's (almost) always better to include the definite article before any possessive adjective: https://www.duolingo.com/comment/5412105