In the united states, where can volcanoes like those at b be found

According to the given diagram, the location B represents an oceanic trench.

When a denser oceanic plate sinks below a less dense continental plate, it results in the formation of a subduction zone and is marked by a deep sea trench. This trenches are very long, narrow and deep and this forms the boundary between a continental and an oceanic plate. The subducting plate undergoes partial melting at greater depth and allows the magma to rise up towards the over-riding plate, giving rise to the formation of volcano or chain of volcanic islands.

Thus, in the figure, B represents a deep-sea oceanic trench.