The correct answer is option 1, Biome. A biome is a large-scale biological community that is characterized by a specific climate, distinct geographical features, and a wide variety of plant and animal species. It covers a vast area of land and can be found in different parts of the world, such as forests, deserts, grasslands, and tundra.

Option 2, Hydrosphere, refers to all the water on Earth, including oceans, lakes, rivers, and groundwater. While the hydrosphere is important for supporting life, it does not encompass all the living organisms and their interactions in an ecosystem.

Option 3, Lithosphere, is the Earth`s solid outer layer, which includes the crust and upper mantle. While the lithosphere plays a role in supporting certain ecosystems, it does not represent the largest ecosystem as a whole.

Option 4, Biosphere, encompasses all the living organisms on Earth and their physical environment. Although the biosphere is essential for supporting life, it is not the largest ecosystem when considering the scale of biomes.