Answer:
As described in is the correct answer.
Step-by-step explanation:
A signal phrase is a phrase or clause that introduces a quotation, summary, or paraphrases something. Signal phrases always require a verb and they also may include the name of the person or the source where the information can be found. Option C, as described, introduces information and after these words (in the passage) it is possible to find the authors, Boyd and Ellison, so it is the correct answer.