Which agency is responsible for overseeing the regulatory authority concerning unreasonably constructed bridges?

The U.S. Coast Guard (USCG) is indeed the agency responsible for overseeing the regulatory authority concerning unreasonably constructed bridges. The USCG's duties include ensuring the safe navigation of waterways, and part of this responsibility involves regulating bridge constructions that may affect navigability. They assess the impacts of bridge design and construction on water traffic, ensuring that these structures do not obstruct vessels and comply with safety standards.

The other agencies mentioned have different mandates; for instance, the Federal Aviation Administration focuses on civil aviation and does not regulate bridges. The U.S. Army Corps of Engineers is responsible for a range of water resource projects, including permitting, but their primary focus is not specifically on bridge regulations. The Division of Water Resources often deals with state-level water management issues, which do not include the regulation of bridges as a primary responsibility. Hence, the USCG stands out as the correct answer for overseeing bridge constructions and navigating safety.