All the following are fertile except

The term "fertile" refers to the ability of soil to support the growth of plants. Therefore, the option that is not fertile would be the one where the soil is not capable of supporting plant growth. Among the given options, "erosional plains" are not fertile because they are formed due to the erosion of soil by wind or water, resulting in soil that lacks the necessary nutrients and moisture for plant growth. In contrast, alluvial plains, flood plains, deltaic plains, and loess plains are fertile due to the deposition of nutrient-rich sediments, which make the soil suitable for plant growth.