The correct answer is - A. They invaded Manchuria.
In the first half of the 20th century, Japan was changing drastically, the country began to militarize intensely, and it also became very hungry for new territories.
China, on the other hand, was in big trouble in this period. The nationalists and the communists had divided the country, and the people were killing each other because of political interests of the elites.
Japan used the weak Chinese state, it launched a quick and strong attack on Manchuria, and managed to conquer it relatively easily. Apart from managing to conquer the region and use its resources, the Japanese engaged into performing a genocide over the local population, which ended with around 20 million deaths.
