Final answer:
The main decision that Haruki, the supply chain manager at UBike, is making regarding the manufacturing network for UBike is selecting potential manufacturing sites in Japan.
Step-by-step explanation:
The main decision that Haruki, the supply chain manager at UBike, is making regarding the manufacturing network for UBike is (B) Selecting potential manufacturing sites in Japan. The passage provided mentions that suppliers to the main manufacturing plant must be within one day's drive of the main assembly point. Therefore, it is crucial for Haruki to select appropriate manufacturing sites in Japan that are strategically located to ensure efficient supply chain management.