Why isn't the answer "nhung cong dan", if it is talking about "the citizens" in plural? Otherwise, doesn't the Vietnamese sentence just mean "That government does not allow a citizen to protest"?
I think it's not talking about the citizens "in plural", but about the citizens "in general", so you don't need những.
According to Google translate, "to protest" is phản đối? biểu tình is not an option there?
Biểu tình is like political protest. Phản đối is like complain protest.