During execution of a construction project (a fish hatchery), operations notified the project manager of a concern that could become an issue but could not be addressed. In which project document should the manager record this concern?

The Risk Register is the correct document because it records potential future events that may impact the project, which is precisely what a "concern that could become an issue" represents. It's a proactive tool for managing uncertainties. A Request for Information (RFI) is used to clarify unclear information, not to record potential problems. The Scope Statement defines the project deliverables and boundaries, not potential issues. An Issue Log is for recording problems that have already occurred and require resolution, whereas this concern is still a potential event.