What is a string of islands formed by the volcanoes along a deep ocean trench called?
A volcanic island arc.
Subduction zones forms the deep ocean trench. As the crust sinks down into the mantle at the subduction zone, the crust is melted. The hot liquid magma that pushes up toward the surface. When the hot liquid magma reaches the surface it forms volcanos.
All along the edges of the ocean trench, subduction zone, liquid magma reaches the surface forming volcanic islands. This string of islands is called a volcanic island arc.