Answer:
 A. form where subduction takes place. 
Step-by-step explanation:
Volcanic arcs - 
It refers to the group of volcanoes , that are formed above the subducting plates , is referred to as volcanic arcs . 
In most of the cases , they are formed from the subduction of an oceanic tectonic plate . 
Various offshore volcanoes form island , referred to as the volcanic island arc. 
Hence , from the given information of the question , 
The correct answer is A.