Explanation: The USA-INT control on a VHF marine radio adapts the channel assignments to comply with regulations for the operating region. Internationally, many VHF channels are designated for *duplex* operation, using separate frequencies for transmit and receive, often for communication with shore stations. However, within U.S. waters, regulations permit or require certain of these channels to be used in *simplex* mode (transmitting and receiving on the same frequency) for direct ship-to-ship communication, enhancing safety and efficiency. Therefore, selecting "USA" mode changes these designated international duplex channels to simplex operation for use in U.S. waters. This makes option C correct. Options A and D are incorrect because they describe the opposite action or a misrepresentation of the primary function. Option B is incorrect because the control's main purpose isn't to *ensure* duplex operation in U.S. waters, but rather to *change* certain channels from duplex to simplex for specific U.S. applications.