Answer:
The answer is letter D.
Step-by-step explanation:
A credible or reliable source refers to an unbiased information that is trusted. The source needs to be "up-to-date" and accurate.
Choice D suggests that Alex Fernandez's article about Chekhov's "The Cossack" is just too outdated for modern criticism. Thus, this makes the choice the best example of challenging the credibility of a source.
In order for sources to be credible, they need to have been published within the last 10 years. Outdated materials are those which have been published more than 10 years ago. Besides this, you also have to know "who" the author is, the purpose of the source (whether it has been proven or not) and the target audience of the source.