Option B: "A successful prince is both merciful and cruel."
The author of a text that discusses the qualities of leadership, particularly one influenced by Machiavellian philosophy, would most likely agree with option B: "A successful prince is both merciful and cruel."
Machiavelli's ideas, as expressed in "The Prince," often suggest that a ruler should balance qualities of mercy and cruelty, depending on the situation, to effectively govern and maintain power. The notion of being both merciful and cruel to adapt to different circumstances is a central theme in Machiavelli's work.