The answer is of course B - water.
Japan is consisted out of four big islands (Honshu, Hokkaido, Shikoku, and Kyushu). While each of them also has their own small characteristics, separate from each other, Japan has in general been isolated due to its specific geographical environment. Not being connected to any landmass such as neighbouring China, the only contact they could have had was by boat. And travelling by boat has always been more difficult compared to packing your stuff and going somewhere.
