The correct answer is option 3: underground water. Although option 1 (oceans), option 2 (rivers), and option 4 (precipitation) may also contribute to the availability of usable water, underground water is considered the main source of usable water on Earth. Underground water, also known as groundwater, refers to the water stored beneath the Earth`s surface in aquifers. It is replenished through processes such as infiltration, where precipitation seeps into the ground, and percolation, where water moves downward through soil and rock layers. Groundwater is an important source of water for drinking, irrigation, and industrial purposes. While oceans contain a significant amount of water, it is salty and not suitable for most purposes without desalination. Rivers and precipitation play a role in the water cycle but do not provide the primary source of usable water.