According to the danish psychologist edgar rubin (1921/2001), when there is an ambiguity of border ownership for images with black and white regions, the border between the two regions is perceived as belonging to:

It's perceived as belonging to one region or the other.
In borders between two nations like this, conflicts often happen because both parties felt that they have the rightful claim to control the region.
If they managed to control the region arounda the bordor, they will most likely able to control the trades.

Ibelieve that the answer is (a).