Something powerful is what italicized word ?

According to stephen hawking in the "origin of the universe" lecture, "why are we here" and "where did we come from" are questions that

Which best compares the authors’ purposes in silent spring and “save the redwoods”? both authors want readers to protect human rights. both authors want readers to focus on saving trees. both authors intend to entertain readers with their essays. both authors intend to convince readers of their viewpoint.

Which of these phrases is an example of understatement?