On the places where there's convergent plate boundary between an oceanic and continental plate, there's a subduction zone. The oceanic plate is denser, heavier, lower than the continental plate. Because of that, the oceanic plate is moving below the continental plate, slowly getting into the mantle where it is melted and destroyed. On the boundary itself, a gap opens up between the plates, a gap that is under constant pressure and geological activity. This gap is basically a valley at the bottom of the ocean, being much deeper than the surrounding area. The term used for these places is an ocean trench, and there's lot of them around the world where there's a convergent plate boundary between an oceanic and continental plate. The best known of these ocean trenches, also being the deepest place on Earth, is the Mariana trench, located on the boundary between the Filipino and the Pacific plates.
B is the best answer here because culture, nationality, history, and ethnicity are all directly tied to one's country (nation). Territory and boundary are not as specific, and state is too narrow.
