Final answer:
The correct answer is A) geosphere, as landslides occur on the solid parts of Earth and are influenced by the hydrosphere (water) and vegetation within the biosphere. Deforestation can worsen this issue by removing natural anchors that stabilize the soil.
Step-by-step explanation:
Heavy rainfall causing a landslide on the side of a mountain is most directly associated with the geosphere. The geosphere refers to the solid parts of Earth, including mountains, where landslides occur. It's important to understand that landslides can be influenced by activities in other spheres, such as the hydrosphere (the water in Earth's system, including rainfall that can trigger landslides) and the vegetation within the biosphere (plant life that can help to stabilize slopes and prevent landslides).
Tree roots, part of the biosphere, help hold hillsides together and therefore help prevent landslides. Deforestation, or the removal of trees for various human activities, compromises this natural defense against soil erosion and can lead to more frequent and severe landslides. Moreover, the interaction between the hydrosphere and the geosphere is a dynamic process that contributes to the shape and stability of the land, such as how heavy rainfall can infiltrate soil and rock, increasing their weight and potentially causing them to slide downhill.