Why are decomposers excluded from the food chain?
The correct option is B. They act at all trophic levels of food chains.
Decomposers feed on organisms of all trophic levels as they feed on their dead and decaying matter. This is why mostly decomposers do not occupy a specific level in the food chain as their source of energy can be an organism at only one specific trophic level. This makes it hard to assign them to a single trophic level in the food chain. As a result, decomposers aren't included in the food chain.
Decomposers play a major role in decaying the organic matter of dead organisms and they have a huge population which makes them relevant at all trophic levels. They can decompose the dead organisms with which they interact directly.