From the given table of definitions:
- influx: movement into an area
- effluent: wastewater discharged into the environment
- reflux: to return or flow back into something
- fluid: capable of flowing or moving freely like water
We can observe a common theme in the definitions. The words influx, effluent, reflux, and fluid all involve the idea of movement or a state of flowing. Hence, the most likely definition of the root flu is:
flow