What government action first allowed American business men to come to China in large numbers

The correct answer is:

Signing of the Burlingame Treaty Apex

The Burlingame Treaty of 1868, signed in Washington was a landmark treaty between the U. S. and Qing China, which ammended the Treaty of Tientsin, (one of the unequal treaties), which established friendly relations between the two nations, with the United States granting China the status of most favored nation in trade.