The correct answer is: Facts
When an author wants to inform it is very important that his oppinion is not given as he is not trying to make the reader's mind change in any way. The objective of this type of writing is to increase knowledge of a certain topic. How and why questions are answered and the best thing to do this is to include facts as they give straigthforward infromation that leaves no chance for ambiguity. As we know, facts are objective as they describe what happens, the information comes from it and the information providen can be compared to reality. For example, if the author states that water boils at 100° Celsius anyone with a thermometer and water can prove it.
