I think the expected solution is résidents.
Here "résident" is used as a common noun. It's important to note that you can also write "résidant" as explained here, but that the Académie Française advises against this spelling.
With a present participle value, it would be used in this way instead:
Ma cousine, résidant au 56 rue Diderot, va bientôt déménager.
My cousin, living at 56 rue Diderot, will soon be moving.
And as a qualifying adjective:
C'est l'appartement où elle était résidante.
This is the apartment where she used to live.
To come back to the question in the exercise, as you've already noticed, after the indefinite article des we mostly place a noun, but sometimes an adjective, as in the sentence Ils sont des sympathiques résidents.
If you could add an image of the exercise in question, perhaps we could be sure that it's an error.