Given what we know about volcanic formations, we can conclude that the Mt. St. Helens volcano was formed by the process described in option D, "Subduction an oceanic continental convergent boundary caused a mountain range to form and magma to come to the surface".
Mt. St. Helens is a volcano that falls under the category of a stratovolcano. Stratovolcanoes are volcanoes that commonly form at subduction zones. This process forms chains or clusters of mountains along the boundaries of tectonic plates, where oceanic crust is pulled under another type of crust. This process tends to form mountain ranges which contain volcanoes, such as:
- Cascade Range (To which Mt. St. Helens belongs)
- Los Andes
- Rocky Mountains
- Campania Mountain range
and so on.
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