The correct option is D (a) and (b) above
Transfer of energy from the producers through a series of organisms with 'repeated eating' and 'being eaten' is known as food chain. The food chain consists of producers, primary consumers, secondary consumers, tertiary consumers or top carnivores and decomposers. Energy trapped by the producers is transferred to other organisms in the food chain. Therefore a food chain in an ecosystem represents both feeding relationship and energy flow. Ecological succession, on the other hand, is the gradual process by which ecosystems change and develop over time.