The correct answer is B: "A podcast that covers one researcher's effort to trace the monarch's yearly journey from the U.S. to Mexico,"
From all the options, a podcast that deals with the subject of the migration of the monarch butterfly would be the best source to help Sinclair write his paper.
This podcast will provide relevant information about the migration patterns of the insects and, at the same time, the data will be given by an especialist who was done thorough research on the subject, making it reliable.