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.