Decomposers are microorganisms that convert dead plants and animals into humus. They play a crucial role in the ecosystem by breaking down organic matter and recycling nutrients back into the soil.

Here is a detailed explanation of why decomposers are the correct answer:

1. Definition of decomposers:
- Decomposers are organisms that obtain their energy by breaking down dead organic matter.
- They are mainly bacteria and fungi, although other organisms such as worms and insects also contribute to the process.

2. Role of decomposers in the ecosystem:
- Decomposers are essential for nutrient cycling in the ecosystem.
- When plants and animals die, decomposers break down their remains into simpler substances, releasing nutrients back into the soil.
- These nutrients are then taken up by plants, which in turn are consumed by herbivores and carnivores, completing the food chain.

3. Process of decomposition:
- Decomposers secrete enzymes that break down complex organic molecules into simpler forms.
- Bacteria and fungi are particularly efficient at decomposing organic matter due to their ability to produce a wide range of enzymes.
- As the organic matter is broken down, it is converted into humus, a dark, nutrient-rich material that improves soil fertility.

4. Importance of decomposition:
- Decomposition helps to recycle nutrients, ensuring that they are available for future generations of plants and animals.
- It also helps to maintain soil structure by improving its ability to hold water and nutrients.
- Without decomposers, dead organic matter would accumulate and nutrients would become locked up, leading to nutrient depletion and reduced productivity in the ecosystem.

In conclusion, decomposers are microorganisms that play a vital role in the ecosystem by breaking down dead plants and animals into humus. They are responsible for nutrient recycling and soil fertility, making them essential for the functioning of the ecosystem.

D) Decomposer