The correct answer is C.
A documentary is the most reliable source of all three.
A blog is a personal account of an event or a description of the author's feelings and ideas. It's extremely subjective and not reliable as a source because the author may make many mistakes in his writing.
An editorial is an opinion-making piece in a newspaper where the editor's opinion is clearly expressed. Again, this is a very subjective piece of writing and based entirely on the writer's personal opinion.
A documentary is always based on actual facts and therefore the most reliable source of information.
