A complete ecosystem includes biological parts and abiotic parts. Biological parts include producers, consumers and decomposers, among which decomposers refer to bacteria and fungi that make a living by decomposing animal and plant remains. Mushrooms belong to fungi, and bugs in ponds are saprophytes. Although earthworm is an animal, it makes a living by decomposing fallen leaves of ground plants and humus in soil, so it also belongs to decomposers.
So choose: D.