The open door policy was challenged by what country with its invasion of manchuria in the 1930s?

The Open Door Policy is a denominate in foreign affairs foremost used to bear to the United States policy established in the late 19th century and the early 20th century, as enunciated in Secretary of State John Hay's Open Door Note, dated September 6, 1899 and transfer to the major European powers.

Ithink the answer should be a.