A boundary between an oceanic plate and a continental plate will be near a coastline., not "far from the coast" or "in the middle of oceans."
Often, mountains are also near that boundary, but mountains may also exist elsewhere, so "anywhere there are mountains" is not an apt description.
The best choice is "on land but close to oceans."
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<em>Additional comment</em>
The 2011 Tohoku earthquake near Japan was offshore by 67 km. It came from the subduction zone where the Pacific plate dives beneath the Eurasian plate. It is an example of a plate boundary quake that occurred offshore, rather than on land.
I believe the correct answer is: Subduction is the collision between two tectonic plates. It is the
biggest crash scene here on Earth. It is a geological process where one
plate moves under another and is forced down into the mantle.
