When it comes to soil richness, the biome with the richest soil is temperate grasslands (option a). These areas, often referred to as prairies or plains, are known for their deep and fertile soils, primarily due to the thick layer of organic matter created by the growth and decay of grasses over time.
The climatic conditions in temperate grasslands, characterized by a moderate climate with seasonal rainfall, promote the growth of grasses rather than large trees, allowing for better soil development. In contrast, biomes like savannas (b) and tropical rain forests (c) may have rich biodiversity but often lack the deep, nutrient-dense soils found in grasslands due to factors like heavy rainfall and leaching. Lastly, tundra (d) is known for its poor soil quality due to extreme cold and limited decomposition of organic material.