Assertion : Decomposers cannot be excluded from the food chains .
Reason : They act all all trophic levels of a food chain .

Options

A. If both Assertion and Reason are true and the Reason is the correct explanation for the Assertion.
B. If both Assertion and Reason are true and the Reason is not the corret explanation of the Assertion .
C. If Assertion is ture the Reason is false .
D. If both Assertion and Reason are false .

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Decomposers cannot be excluded from the food chains." This means that decomposers play a crucial role in the ecosystem and are essential for the functioning of food chains.

2. **Understanding the Reason**: The reason provided is that "They act at all trophic levels of a food chain." This indicates that decomposers interact with organisms at various levels of the food chain, including producers (plants) and consumers (herbivores and carnivores).

3. **Role of Decomposers**: Decomposers, such as bacteria and fungi, break down dead organic matter from both producers and consumers. This process is vital for nutrient cycling, as it returns essential nutrients back to the soil, allowing plants to grow and continue the cycle of life.

4. **Nutrient Cycling**: Without decomposers, dead matter would accumulate, and nutrients would not be recycled. This would lead to a depletion of nutrients in the ecosystem, ultimately affecting all trophic levels.

5. **Conclusion**: Both the assertion and the reason are true. The assertion is correct because decomposers are integral to food chains, and the reason is also correct as it explains how decomposers function at all levels of the food chain.

6. **Final Answer**: Therefore, the correct option is that both the assertion and the reason are true, and the reason is a correct explanation of the assertion.