Final answer:
The true role of earthworms in soil formation includes breaking down rocks into soil, aiding water retention, and enhancing nutrient cycling. Thus, the answer is d. All of the above.
Step-by-step explanation:
Charles Darwin's study, titled 'The Formation of Vegetable Mould Through the Action of Worms,' explores the critical role of earthworms in soil formation, which encompasses a variety of ecological functions. Earthworms are instrumental in the breakdown of organic matter, which is crucial to enhancing nutrient cycling and to the formation of humus. They also improve soil aeration and structure by creating a network of burrows, which helps soil retain more water and promotes better root growth for plants.
The true role of earthworms in soil formation includes: breaking down rocks into soil, aiding in water retention, and enhancing nutrient cycling. Therefore, the correct answer to the student's question is d. All of the above. Each of these aspects is interconnected, contributing to the overall fertility and sustainability of soil ecosystems, impacting plant growth, and, by extension, all terrestrial life that depends on plants for sustenance.