Final answer:
The recent earthquakes in Washington's Puget Sound region were caused by movement of the Juan de Fuca Plate.
Step-by-step explanation:
The correct answer is C) Juan de Fuca Plate.
The Puget Sound region in Washington experiences frequent earthquakes, and these earthquakes are primarily caused by the movement of the subducting Juan de Fuca Plate. The Juan de Fuca Plate is a small tectonic plate located off the coast of Washington and Oregon. It is subducting beneath the North American Plate, which is responsible for the formation of the Cascade Range, including Mount St. Helens, through mountain-building processes.