Step 1: Concept Identification: The objective is to pinpoint traditional rainwater harvesting systems specific to Bihar.
Step 3: System Analysis: Associating each system with its region:
\[\begin{array}{rl} \bullet & \text{(A) Eris: Tank irrigation systems prevalent in Tamil Nadu.} \\ \bullet & \text{(B) Ahar: Rectangular embankment reservoirs for runoff storage, traditional to South Bihar.} \\ \bullet & \text{(C) Surangams: Horizontal wells on hill slopes, found in Kerala and Karnataka.} \\ \bullet & \text{(D) Pyne: Artificial channels diverting river water to fields. The "Ahar-Pyne" system is a recognized floodwater harvesting method originating in ancient South Bihar.} \\ \end{array}\]
Thus, both Ahars and Pynes are utilized in Bihar.
Step 4: Conclusion: Ahars and Pynes are the traditional rainwater harvesting systems commonly employed in Bihar.