Final answer:
The 2011 B prompt by Sam Roberts uses the mode of argumentation as it presents reasons and evidence to convince the reader of the need to transition away from fossil fuels.
Step-by-step explanation:
The prompt from Sam Roberts discussing the United States' journey away from fossil fuels and looking at alternatives appears to use a mode of argumentation. Argumentation involves presenting a set of reasons or evidence in support of a conclusion. In this case, the text presents various scenarios, discusses the limitations and impacts of fossil fuel usage, and explores potential solutions and alternatives. This mode goes beyond merely explaining or describing the situation (exposition and description) and aims at convincing the reader about the urgency and necessity to transition away from fossil fuels, hence the argumentative purpose behind the presented facts and analysis.