Answer:
At the site of convergent plate boundary
Step-by-step explanation:
The trenches are the narrow and deep V-shaped depression that is formed at the convergent plate boundary, where the heavier plate subducts below the less denser plate. This feature marks the presence of a subduction zone, that lies just below the trench. This trenches usually borders an island arc or a continent.
The deepest of all the trench is the Mariana trench which has a depth of nearly 11 km.
Thus, the trenches are formed at the site of convergent plate boundary, where one plate goes below the other.