The correct answer of "2 and 3 only" is appropriate because decomposers, which include fungi and bacteria, play a crucial role in ecosystems by breaking down dead organic matter and recycling nutrients back into the soil. This process is essential for maintaining soil health and supporting primary producers, while viruses do not fit into this category as they generally do not decompose organic matter but rather infect living organisms. Understanding the roles of decomposers is vital in ecological studies, as articulated in UPSC-level resources like NCERT textbooks, which emphasize their importance in the food chain and nutrient cycling.