Which of the following does not occur at a subduction zone?
a. the leading edges of both plates are bent downward.
b. oceanic crust is pushed down into the mantle.
c. one oceanic plate moves into another oceanic plate.
d. one continental plate moves into an oceanic plate.

The correct answer is option A, that is, the leading edges of both plates are bent downward.

Explanation:

A subduction zone refers to the biggest crash scenario on Earth. These boundaries signify the collision between two of the tectonic plates found on the planet Earth. The plates refer to the components of crust, which gradually passes across the surface of the planet over many years.  

When two of the tectonic plates comes in contact at a subduction zone, one slides and bends beneath the other, moving down into the mantle. Tectonic plates can mediate both oceanic and continental crust, or they may be formed of only one type of crust. Being denser than continental crust, the oceanic crust generally sinks into the mantle underneath the lighter continental crust at a subduction zone.