Final answer:
The most credible source for a literary argument on 'To Kill a Mockingbird' is Rebecca H. Best's article from The Mississippi Quarterly because it is a peer-reviewed journal article that provides deep analysis of the characters and themes relevant to forming a solid argument.
Step-by-step explanation:
When selecting the most credible source and useful information for a literary argument paper on To Kill a Mockingbird, it's important to evaluate the credibility of each excerpt provided. The passage from Wikipedia, while providing factually accurate information about the novel's success, lacks in-depth analysis and may not be sufficient for a literary argument.
The excerpt from www.gradesaver.com offers an interpretation of the novel's symbolism and thematic elements related to the children's perspective and could support a literary analysis. However, the most credible source is Rebecca H. Best's article from The Mississippi Quarterly, accessed through the Literature Resource Center database.
This peer-reviewed journal article discusses the depth of the novel's characters and their interactions with the concept of 'the Other,' thus providing a rich source for constructing a literary argument.