The source of most irrigation and drinking water is groundwater , which is water stored underground in soil and rock layers called aquifers. A bit of context for your post structure:

  • Globally, groundwater is the most accessed source of freshwater for people and farms, supplying around half of the world’s drinking water and roughly 40% of irrigation water, as well as a large share of industrial use.
  • Aquifers store the vast majority of the planet’s liquid fresh water, and in many countries they provide the primary water for both city taps and rural wells.

So if the question is:
“the source of most irrigation and drinking water is ________”
→ the correct fill‑in is: groundwater (aquifers).