"Who are the people painting an owl?"
Translation:Ketkä maalaavat pöllöä?
Perhaps Keitä ovat ihmiset, jotka maalaavat pöllöä?
'People' is the subject of the sentence, so I think it needs to be nominative. It's also plural, so nominative plural ihmiset.
The verb needs to agree with the subject, so ovat.
I think 'who' is the predicative, and so far, every time I've seen a plural subject, its predicative has been partitive plural. Thus keitä.
And if ihmiset is plural, the relative pronoun also needs to be plural, so jotka.
First time the correct answer demands "Who are the people painting..." but the second time "Who are painting..." It doesn't make any sense! Also, I am a native speaker, admittedly not spoken Finnish for decades, and I would not usually say who are the people (unless perhaps I want to emphasize that there are different groups of people).