The MOST SIGNIFICANT cost involved in converting the land back into a forest would be:
B. The farmers must be compensated for the loss of farmland.
Compensation for farmers is a critical factor because it addresses the economic implications of taking productive land out of agriculture, which can be a highly contentious issue. Ensuring that farmers are fairly compensated can help gain their support for the reforestation effort and mitigate conflicts over land use. The other options, while important, do not encompass the immediate economic and social impacts that come with converting farmland back to forest.