The correct answer is: Groundwater.
Groundwater is the water that fills the cracks and pores in soil and rock beneath the Earth’s surface. It is a major source of irrigation water for agriculture, and it is also used for drinking, bathing, and other domestic purposes.
Rivers and lakes are also sources of irrigation water, but they are not as reliable as groundwater. Rivers and lakes can dry up during droughts, and they can also be polluted by human activities.
Desalination plants are used to remove salt from seawater, but they are expensive to operate. They are also not a practical option for many areas, as they require access to the ocean.
In conclusion, groundwater is the most reliable source of irrigation water. It is a renewable resource that is not subject to the same fluctuations as surface water. Groundwater is also a relatively clean source of water, and it is available in many parts of the world.