Currently? probably overpopulation
The primary issue with a one party system is the suppression of individual rights and no democratic rights.
In a one-party system, like the one in Communist China, the people have no voice in how the country is run. One political party makes all the decision without any opposition in the parliament.
From the perspective of human rights, this will lead to dictatorship and a curb on freedom of speech.
One reason why Japan has such a distinctive culture is because it was closed off from the world from 1623 to 1641. The Tokugawa Shogunate closed Japan because they were afraid of all the new ideas and cultures that came from trading. Because of this period of isolation, Japan created their own unique culture without being influenced by any western ideas.
Hope this helps! Please let me know if I'm wrong :)
I believe A. I am not 100% sure but that is my best guess.