C is the best answer to this question.
Choice A is not likely to be an answer because an opinion is biased information, and we are looking for a credible source, not a biased one. B is incorrect because something that is interesting does not automatically mean it's credible (for example, fictional novels). D is incorrect because something that is recommended by a friend cannot always be credible. For example, a friend could recommend a biased article, which is not credible.