Where it is unclear in the English whether the answer should be singular or plural I have been reporting it, suggesting both should be accepted. It might be a good idea for you and others to do the same, so that between us we can catch more of them.
This question does not indicate singular or plural, therefore both answers are correct. But because this particular lession is about plurals, Duolingo expects your answer to be in the plural. We must always keep in mind what the lession is about.