I was asked to fill in the correct word from a list that included both. It seems that you can design a valid translation question, which somehow becomes a 'fill in the gap' question with no clue as to what meaning was in the composer's mind.
Does Duolingo simply pick similar-looking words from its database (which is bound to fail) or does a human choose the range of options, in which case you have to think carefully from the point of view of someone who does not know what you were intending?
As I explain in response to Joanne, it was a 'click the correct box' question and they do not seem to accept multiple answers - they would be a bit pointless if they did. If I remember correctly, cat and cait were the only options.
I use the website. For some unknown reason this means I cannot furnish any further details when I report 'something else is wrong'. And if I simply said that cait was right, without my reasoning, you would not know what I was on about.