Based on the table of definitions, the most likely definition of the root "flu" is "flow".
This conclusion can be made by relating each word to its definition:
1. Influx: Movement into an area – This implies a flow of something entering an area.
2. Effluent: Wastewater discharged into the environment – This involves the flowing out of wastewater.
3. Reflux: To return or flow back into something – This directly references the concept of flowing back.
4. Fluid: Capable of flowing or moving freely like water – This clearly involves the ability to flow.
The common theme among all these definitions is the concept of "flow." Therefore, "flow" is the most likely definition of the root "flu."
