Correct option is C) flows
Option D is incorrect because will be flowing means that the river will flow later on, which is incorrect because it has been flowing.
Option B is incorrect because flown is the past tense of flow, which means that it used to flow but we don’t know if it flows now, which is incorrect.
Option A is incorrect because although isn’t flowing Ian in the present tense, it means it is flowing in the present, and it probably didn’t flow before or won’t flow later, which is totally incorrect.
Option C is the correct answer because ‘flows’ means that the water in the river flows regularly, hence it is the correct answer.