Why was the open door policy important to the united states?
(a) it gave the united states colonial territory in asia.
(b) it ensured the united states would have access to chinese markets.
(c) it locked european nations out of chinese markets.
(d) it allowed china to invest in the united states.
(e) it further secured the legitimacy of the monroe doctrine.

The answer is B

Basically, Europe (Britain, Germany, Russia, France, etc.) was trying to divide up China among itself. America (since they now had the Philippines and could trade more in Asia) wanted to make sure they weren't locked out of trading with China. So the US kinda stepped into the discussion and said "I think that no matter what happens, let's not stop anyone from trading here. So even if Russia or Britain hates us at some point, that doesn't stop us from doing what we need to do."

What's funny is most of Europe went along with it. (except Russia because Russia don't care).