The key difference between the invasions of China and Poland is encapsulated in option C: "The invasion of Poland started World War II."
While Japan's invasion of China (which began in earnest in 1937) contributed to the escalation of tensions in East Asia and set the stage for broader conflicts, it was the Nazi invasion of Poland on September 1, 1939, that is widely recognized as the event that triggered World War II in Europe.
To briefly evaluate the other options:
A is incorrect; the invasion of Poland was relatively quick, completing in a matter of weeks, while Japan's conflict in China became protracted and continued for years.
B is partially misleading; while there was some international condemnation of Japan's actions in China, the major Western powers did not intervene significantly to stop it at that time. Conversely, the invasion of Poland did lead to immediate military responses from Britain and France.
D is incorrect; the invasion of China did not start World War II.
Therefore, option C is the correct answer.