The correct answer is - fertile soil.
Nepal and Bhutan, unlike the other countries of South Asia, lack in fertile soil as a natural resource. That is due to the mountainous terrain and high altitudes where these two countries are located, so the climate and the steep terrain are not allowing the formation of fertile soils in this part of the region.
Because these two countries lack fertile soil, they are not able to produce large quantities of food, so they are heavily dependent on import of food from the other countries in the region, which puts them in a very bad position, as they are directly dependent on the other countries for their survival.
