Food chain is the process or method that connects plants and animals to the ecosystem. In this process, herbivorous animals feed on plant for their nutrition and the carnivorous animals feed on the herbivores as their source of food.
Soil plays a key role in this process as it contains many molecules like phosphorous, nitrogen etc. which is taken up by the plants through roots and hence consumed by the animals. This is how energy ascends from one trophic level to another although there is a decrease in each increasing trophic level.
it's c. (to reduce the greenhouse effect/carbon gasses)