Explanation: "EOS" in a Digital Selective Calling (DSC) message stands for **End Of Sequence**. DSC is a crucial global standard for automatically establishing radio calls, particularly for distress, urgency, and safety communications. Its messages are highly structured, using specific digital codes to convey information like the nature of a distress, position, and the identity of the calling station. The EOS code serves as a vital control character that definitively marks the conclusion of a DSC message. This allows the receiving DSC equipment to recognize that the entire message has been received, validate its integrity, and then correctly process the information contained within. Without EOS, the receiver wouldn't know when the message was truly complete, leading to potential misinterpretations or truncated data. Options A ("Error Of Sequence") and D ("Equal Operating Signals") are not recognized terms within DSC protocols. Option B ("End Of Signals"), while conceptually similar, is not the precise terminology used in the international ITU-R M.493 standard that defines DSC message structure.