"Do you need a broom?"
Acceptable answer? : Mnahitaji ufagio.
I wrote this and got the question "right" but it marked it as a typo for "Unahitaji..."
Yeah, mnahitaji is also right. There's no context to help you know if it's singular or plural in English (except maybe the pragmatic information that brooms are generally operated by one person ... but a group of people can still need "a broom"), so both should be accepted.
Unfortunately, Duolingo accepts "typos" where one letter is wrong, so there isn't even a way to report this.