The primary reason the united states advocated the open door policy was to a. protect and broaden u. s. trade with china. b. expand the concept of the monroe doctrine to asia. c. justify our taking the philippines as a us colony. d. justify the extension of american colonies into the far east.

The correct answer is A
a.protect and broaden u.s. trade with china.
 
The doctrine supported the territorial integrity of China while also calling for chinese authorities to treat all international powers equally in the area of trade,such that it would not impose tariffs unfairly. The policy was proposed by secretary Hay in 1899.

Both countries had to aprove annexation.