Which letter is used to designate the Rocky Mountain Region?

The designation for the Rocky Mountain Region is represented by the letters "RM." This abbreviation effectively conveys a widely recognized reference to the geographic area characterized by its mountainous terrain and distinct regional features.

Using the letters "RM" enables clear and concise communication regarding this specific region, especially within contexts where multiple geographical regions are being discussed. The choice of letters not only aligns with common naming conventions found in geographic designations but also helps avoid confusion with other regions, thereby facilitating more effective information sharing and regional identification.

In comparison, the other options represent different regions or contexts that do not pertain to the Rocky Mountain Region, which solidifies "RM" as the most appropriate and accurate choice for this designation.