Which of the following is not a type of mission message?

The correct answer is 'Optional' because this category does not align with the recognized classifications of mission messages within military communications. Mission messages are typically categorized into specific types that reflect the urgency and importance of the communication.

Immediate messages are critical and require urgent attention, often pertaining to time-sensitive operations or safety situations. Routine messages are standard communications that are not urgent but still necessary for maintaining operations and logistics. Priority messages fall in between; they are important and should be addressed promptly, but they do not possess the immediate urgency of the immediate type.

In contrast, 'Optional' does not fit into these established categories, as it suggests a non-essential nature that doesn't resonate with the structured approach of military communications. Therefore, 'Optional' is the classification that is not a recognized type of mission message.