Can "Cei" not mean "Okay" ? It sounds very similar and has the same meaning here, but is marked incorrect.
'Cei' is the positive response to a question starting with "ga i?' (May I) and thus it should be translated as either 'Yes' or 'Yes, you may' or occasionally 'You may'.
If the response is to be 'Okay' or 'OK' then something like 'iawn' or 'o'r gorau' would be more appropriate.
I though 'ga i' was 'can I have'. This example doesn't involve possession at all but asking, where does the 'can I have' part fit into it?