Which of the following is the largest available source of fresh surface water

Groundwater is the largest source of freshwater.

Explanation:

According to a study conducted by Boswinkel et al., (200), groundwater is the largest source (~90%) of freshwater followed by lakes and rivers. Furthermore, according to United Nations Environment Programme and World Bank statstics, ~1.5 billion people depends on groundwater for their drinking water needs around the globe. Likewise, on yearly basis, groundwater is withdrawn at 600-700 km3 which is approximately 20% of global water withdrawals.

oxygen +1 most likely