Analogy — Controller and controlled flow/system: Signal : Traffic :: Dam : ? Pick the system/flow that is regulated by the left term, as traffic is regulated by a signal.

Introduction / Context:
A traffic signal regulates the movement of traffic. A dam regulates the flow of a river. The relation is “regulator → regulated system/flow”.

Why Other Options Are Wrong:

• Lens : Light — Modifies/focuses, but not a traffic-like regulatory control analogy.
• Door : House — Access barrier to a place, not flow control of a medium.
• Operation : Doctor — Reversed roles; doctor performs operation.
• None of these — invalid since a correct pair exists.

Final Answer:
Dam : River