The correct answer is - Pacific plate.
The North American tectonic plate is bordering the Pacific tectonic plate. They form a convergent plate boundary, where the heavier oceanic Pacific plate is subducting bellow the lighter continental North American plate.
As the Pacific plate started to subduct below the North American plate, it also started to lift up the surrounding area with all of the pressure from bellow. The land was gaining on elevation continuously for millions of years, and gradually the Rocky Mountains have been formed i the shape and size we now know them.