The most accurate response regarding Japan's main reason for territorial expansion in the twentieth century is:
Japan needed natural resources to provide for its growing population.
During this period, Japan sought to expand its territory to access essential natural resources, such as coal, iron, and oil, which were crucial for its industrial growth and modernization. The country's limited natural resources made it increasingly reliant on imports, leading to the need for expansion to secure these vital resources to support its burgeoning population and rapid industrialization.