The correct answer is - It is the part of the ocean where new crusts are formed.
The place marked with A on the map is the place in the ocean where the new crust is formed. That is a place where there's a divergent plate boundary, or rather a place where the tectonic plates are moving away from one another. The gap and cracks left between them are easy target for the magma from the mantle to penetrate towards the surface. As the magma reaches the ocean floor it starts to cool off very quickly, creating new crust, and slowly making a very large underwater mountain range known as mid-ocean ridge.
Explanation: In the given picture, we could see that structure A is an underwater sea mount, where continuous erosion has taken place by the ocean water at a time when the sea level was at a lower height.
This continuous erosion of the top of the sea mount depicts that a part of the previously existing crust has been eroded and after a certain period of time, again the sea level had rise upto some extent. This has no relation to the subduction zone.
