The correct answer is C) By its call sign. Coast radio stations communicating by HF radio normally identify themselves by their call sign, as required by international radio regulations. Call signs are unique identifiers assigned to each radio station to facilitate identification during communications. This is the standard practice for all maritime radio stations, including those operated by the US Coast Guard, to ensure proper identification during transmissions. The other options are incorrect because a subscriber number is not a standard means of identification for a coast radio station, MMSI numbers are used for digital selective calling but not for voice communications, and MID (maritime identification digits) are used to identify the country of registration but not the specific radio station.