Question

1. a subduction zone is formed...

at every type of divergent plate boundary
when one tectonic plate sinks below another
at every type of convergent plate boundary
at transform boundaries

Explanation:

A subduction zone is a region where one tectonic plate moves under another. Divergent plate boundaries are where plates move apart, not where subduction occurs. Convergent plate boundaries can have different types like continent - continent collision (no subduction in some cases). Transform boundaries are where plates slide past each other. So, the key is the definition of subduction (one plate sinking below another).

Answer:

When one tectonic plate sinks below another