Explanation: NAVTEX receivers are designed to allow users to filter incoming messages based on their relevance. The second letter (B2 in the B1B2B3B4 message header format) indicates the specific type of message, such as navigational warnings, meteorological forecasts, search and rescue information, or pilot service messages. Users can program their receiver to reject certain message types they deem irrelevant, preventing them from being displayed or triggering an alarm. However, some critical message types (e.g., navigational warnings, meteorological warnings, and SAR information) are designated as non-rejectable to ensure vital safety information is always received. A) The transmitter identity (B1) allows users to select which transmitting stations they want to receive messages from, not to reject specific message *types* from an already selected station. C) NAVTEX is a broadcast system; there is no "answerback" from the receiving station to the transmitter. D) Messages with serial number 00 are typically urgent safety-related messages and are often non-rejectable, meaning they are always received, not rejected.