The correct answer is D) Federal Communications Commission. The Federal Communications Commission (FCC) is responsible for regulating the frequencies used for bridge-to-bridge radiotelephone communications. This is because the FCC has the authority to manage and allocate the radio frequency spectrum, including the specific frequencies used for maritime communications. The other options (Department of Defense, Department of Transportation, and U.S. Coast Guard) are not responsible for setting the regulations on the frequencies used for bridge-to-bridge radiotelephone, as this falls under the jurisdiction of the FCC.