"That one is correct."
Translation:Tuo on oikein.
It depends. But I should have asked the other way around. It is rather common to say 'That is correct' in English. If you add the 'one' to the question, it can be meaningless, but it can also mean that you want to focus on the 'one' part. Like 'That one is correct, but all the other stuff is wrong.' So it might be less confusing at this stage to ask for a translation of 'that is correct'. That way, beginners like me don't have to wonder about what to do with the 'one' part of the sentence.
I agree. You could say "That is correct" and be referring to an entire paper. Where as "That one is correct" would be referring to a specific Question/Answer. There are also several other phrases in this lesson that Require yksi to be used when there is "one" in english. So it can get confusing when those don't accept answers without yksi. and this doesnt accept an answer with it.
Yeah i dont know why duolingo uses yksi in these lessons because its not something ive ever come across in real life. Yksi is rarely ever used in finnish, which is something that drives me insane every once in a while, but unless its a specific amount of something id recommend never using it.