Which characteristic best defines a reliable source in nonfiction writing?

A reliable source in nonfiction writing is best defined by being authored by an expert in the field. When a source is created by someone with proven knowledge, credentials, or significant experience in a particular subject, it lends credibility to the information presented. Experts are typically recognized for their authority and understanding of the topic, which leads to a greater trust in the accuracy and reliability of their insights.

While other characteristics, such as length or style, can have their own merits in some contexts, they do not inherently indicate reliability. Lengthy and complex texts might provide depth but are not necessarily accurate or trustworthy; a narrative style can engage readers but does not guarantee factual correctness; and anecdotal evidence can be persuasive but lacks the empirical support that often strengthens an argument. Therefore, the presence of an expert author fundamentally enhances the trustworthiness and reliability of a nonfiction source.