Which feature marks the boundary where one oceanic plate sinks beneath another?

• A. Trench
• B. Island arc
• C. Subduction zone
• D. Magma formed

Answer: (C)

The key idea is the boundary where one oceanic plate sinks beneath another, which is called a subduction zone. This term names the active zone of interaction where the denser plate dives into the mantle under the lighter plate, driving deformation, earthquakes, and volcanic activity. The trench is a prominent surface feature you see at many subduction zones, marking the edge where the descending plate bends, but it’s the boundary itself that’s defined as the subduction zone. Island arcs form above this area from the melting of the subducting slab and the rise of magma, so they’re a consequence of subduction rather than the boundary name. Magma formation describes what happens as the slab melts and feeds volcanism, not the boundary that defines where sinking occurs.