The answer is D: their tone is always objective.
When writing nonfiction, the author should always use an objective tone in order to convey an idea, or set of ideas, in formal language, trying to achieve precision, conciseness, accuracy and demonstrability (this latter characteristic entails that any claim made in the piece of writing can be corroborated by anybody else). Objectivity and formal language are aimed at avoiding ambiguities and miscomprehension.
