What present-day country lies to the east of Japan,
across the Pacific Ocean

East Sea or Sea of Japan (same name but different versions)

Explanation:

"Sea of Japan, Japanese Nihon-kai, Russian Yaponskoye More, also called East Sea, Korean Tonghae or Donghae, marginal sea of the western Pacific Ocean. It is bounded by Japan and Sakhalin Island to the east and by Russia and Korea on the Asian mainland to the west. Its area is 377,600 square miles (978,000 square km)" (Britannica).

nothing certain is known about what happened, but it seems that the country declined gradually, perhaps due to bad government, or, more likely, due to a series of bad harvests, which resulted in widespread and recurrent famine.

the correct answer is b