Explanation: NAVTEX (Navigational Telex) is an international automated direct-printing service for transmitting maritime safety information. It operates by broadcasting messages in specific categories, each designated by a single letter identifier. For example, 'A' indicates navigational warnings, 'B' signifies meteorological warnings, and 'D' is for SAR (Search and Rescue) information. Shipboard NAVTEX receivers can be programmed by the user to accept or reject certain message categories, ensuring mariners only receive the information relevant to their operations. The other options are incorrect because NAVTEX is a continuous, automated broadcast system. It does not operate on request (C), its timing is not tied to radiotelephone traffic lists (B), nor is it dictated by radiotelephone silent periods (D). Its broadcast schedule is independent and designed for automatic reception of categorized safety information.