The correct answer is d) fungi.
Fungi play a crucial role in the ecosystem as decomposers. They break down dead organic matter, returning nutrients back to the soil, which is essential for plant growth. While protists, animals, and plants contribute to various ecological functions, it is fungi that specialize in the decomposition process. They can decompose complex organic materials, such as cellulose and lignin, which many other organisms struggle to break down. This makes fungi fundamental to nutrient cycling in ecosystems.