"Is our mayor corrupt?"
Translation:Ist unser Bürgermeister korrupt?
There are two correct options, from which we could choose either. They are:
Unser Bürgermeister ist korrupt - for a male mayor
Unsere Bürgermeisterin ist korrupt - for a female mayor
If you instead write, incorrectly, 'unsere Bürgermeister ist korrupt' then Duolingo marks you wrong and offers one of the above suggestions instead. In my experience, when the article doesn't match the noun Duolingo usually assumes you used the right article and suggests a matching noun for it, rather than (possibly more usefully) correcting the article.