Which word best describes a command or order that is extremely necessary?

The term that best describes a command or order that is extremely necessary is "imperative." This word conveys a sense of urgency and importance, indicating that the command must be followed and cannot be overlooked. It emphasizes the essential nature of the action required, implying that there could be significant consequences if it is not executed.

In contrast, other terms like "voluntary" suggest that the action is optional and based on personal choice, while "optional" and "discretionary" imply that there is flexibility or freedom in deciding whether or not to act. None of these terms carry the same weight of necessity that "imperative" does, making it the most fitting choice in this context.