Answer:
C. The author's credentials.
Step-by-step explanation:
The credibility of a health source here(among these options) can only be checked through the author's credentials as the other options seem irrelevant to the context of credibility or reliability of a health source. The author's credentials would help in knowing the authenticity of the source by analyzing that whether the writer has any belonging to the medical healthcare or he is just merely imitating some other source. Thus, among these options, the author's credentials would help to check the authenticity of the source one is reading or exploring.