What primarily denotes expertise when assessing a source’s credibility?

The correct choice centers on the importance of credentials and objective knowledge when determining the credibility of a source. Expertise is often established through formal education, licenses, certifications, and specialized training that demonstrate a person's qualifications in a specific field. These credentials provide assurance that the source has the necessary understanding and skills to provide reliable information.

Objective knowledge further complements this by indicating that the individual has a well-rounded comprehension of their subject matter, not just a singular perspective or anecdotal experience. In contrast, while the advice and opinions of colleagues might offer insight, they do not independently validate expertise. Popularity and community feedback can indicate a source's reach or acceptance but do not necessarily equate to the depth of knowledge or formal qualifications. Length of experience in a field can be beneficial, but without the necessary credentials or deeper understanding, it does not alone guarantee expertise. Therefore, focusing on credentials and objective knowledge is the most reliable way to assess a source's credibility.