b. What type of tectonic plate boundary exists at the Mariana Trench?

b. The type of tectonic plate boundary that exists at the Mariana Trench is a convergent plate boundary, specifically one involving subduction. The text explains that convergent boundaries occur when two tectonic plates collide, and in the case of the Mariana Trench, the denser Pacific Plate is forced under the Mariana Plate.