Final answer:
The plate to the west and directly adjacent to the plate on which Montana is located is the Pacific Plate.
Step-by-step explanation:
The plate to the west and directly adjacent to the plate on which Montana is located is the Pacific Plate. The Pacific Plate is moving northward relative to the North American Plate. This movement is responsible for the San Andreas Fault in California, where the Pacific Plate and the North American Plate meet.