Final answer:
The revision clarifies the League of Nations' actions by directly mentioning the harsh sanctions and their role in contributing to the conditions leading to World War II.
Step-by-step explanation:
The BEST revision of the underlined sentence in the paragraph "A policy was set forth by the League of Nations following World War I, an extremely grievous policy towards Germany, of blaming Germany that was responsible for the volatile conditions that led up to World War II." could be: "Following World War I, the League of Nations imposed harsh sanctions on Germany, including assigning war guilt and demanding reparations, which contributed to the unstable conditions precipitating World War II." This revision provides clarity by directly identifying the sanctions as harsh, specific actions (assigning war guilt and demanding reparations) taken by the League of Nations, and describing their effect on the pre-war environment.