Final answer:
Mollisols are the soils of grassland ecosystems, characterized by a thick, fertile A horizon high in organic matter, excellent for agriculture, and typically found in regions like the Great Plains.
Step-by-step explanation:
The correct answer to the question regarding the soils of grassland ecosystems characterized by a thick, dark surface horizon known as mollic epipedon is b) Mollisols. These soils result from the long-term addition of organic materials derived from plant roots and primarily occur in the middle latitudes and extensive prairie regions, such as the Great Plains. The Mollisols are described as having a thick A horizon rich in organic matter due to the significant contribution from grasslands' roots and rhizomes, making them very fertile and excellent for agriculture.
Mollisols are typically found in regions with moderate temperatures and rainfall, ranging from 25 cm to 75 cm (9.8-29.5 in) annually. These grasslands support a dominant vegetation of grasses, which contribute to the dense and fertile soil by replenishing the organic material when their roots and rhizomes die and decay. This ecosystem is maintained by a balance of low precipitation, frequent fires, and grazing.