Final answer:
The correct analogy to 'Flow : River' is 'Stagnant : Pool' because while a river flows continuously, a stagnant pool of water is motionless and can accumulate algae and microorganisms.
Step-by-step explanation:
The analogy presented in the question Flow : River :: Stagnant : ? is one that compares the characteristic of movement in water bodies. A river is characterized by its continuous flow, constantly moving water that carries large amounts of water from the source to a larger body of water like a lake or ocean. In contrast, stagnant water is water that is not flowing; it is often motionless and can become stale or dirty over time. Therefore, the most appropriate answer to the analogy is 'Pool,' as in a stagnant pool of water, which typically does not have a current and can become a breeding ground for algae and other microorganisms, similarly to the algal bloom observed in an uncontrolled growth in lakes.