What would most likely increase the carrying capacity for humans in a certain region?

a. new source of fresh water

Other choices are:

b. less access to medicine 

c. a higher birthrate 

d. a higher fertility rate

New source of fresh water would most likely increase the carrying capacity for humans in a certain region. Water plays many important roles in the ecological and environmental systems, and also in humans. In cellular activities and organismic activities, water is used primarily.
For example,
Water is produce when glucose and fructose undergo a condensation process. The water is removed by the combination of hydrogen and a hydroxyl together. Glucose and Fructose forms a substance called glycosidic linkage. And hydrogen and hydroxyl is separated from glucose and fructose. When Hydrogen and hydroxyl is combined, they create H2o or water.

They create genetic variations