I think it could also mean "Answer well" and through this example it is shown that you have to follow the context to differ whether it is a command or a noun
I think there should be a vi before zuri for that to work, IIRC this vi turns adjectives into adverbs.
Good point! Then this answer is grammatically correct but in the wrong lesson (commands). Maybe they are just trying to keep us on our toes.
Why does -zuri not need a prefix here? If jibu in this context is a noun not a command.
Because jibu is in the JI/MA noun class (5/6), and class 5 nouns are the only ones that take a bare adjective with no prefix.
jibu zuri (good answer)
majibu mazuri (good answers)
Take a look at the table on the second page of this adjectives lesson: https://www2.ku.edu/~kiswahili/pdfs/lesson_24.pdf