Which security classification would you apply to a standing operating procedure SOP in which the unauthorized disclosure of its contents could reasonably be expected to cause exceptionally grave dam?

The security classification that should be applied to a standing operating procedure (SOP) with such potential consequences would be "Top Secret." This classification is reserved for information whose unauthorized disclosure could reasonably be expected to cause exceptionally grave damage to national security. It is important to ensure that access to this SOP is restricted to individuals with the appropriate clearance level to protect sensitive information.