Explanation:

- The passage describes Bhutan's natural, organic farming practices and emphasizes the local population's reliance on agriculture.

- The missing word should convey why the population engages in farming.

- Provision: Supplying or providing something, often related to storing or preparing food.

- Conservation: Protection or preservation of resources. Doesn't fit the context of personal or community farming motives.

- Nutrition: Refers to obtaining the necessary food for health and growth. While relevant, it's not directly about the motivation for farming.

- Sustenance: The means of supporting life, especially by obtaining food and resources for daily living. It captures the idea of farming to maintain life and livelihood.

- Therefore, "sustenance" is the best fit to explain why the majority participate in farming.

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