Which single source of freshwater represents the largest volume of freshwater worldwide readily available to use by humans

The correct answer is - groundwater.

The freshwater is represented with only 3% of the total water reserves on the planet. Further more, most of the freshwater is actually frozen in the ice caps and glaciers on the poles and their close proximity, and that water accounts for around 87%. The rivers and the lakes do provide freshwater, but they only account for 1% of the total freshwater reserves, not to mention the fact that big portion of them are heavily polluted and can not be used for drinking at all.

The groundwater though, is much less polluted, is easy to extract and to use, and it accounts for around 12% of the total freshwater reserves, which makes the prime candidate for usage.