The correct answer is B) it ended French control of the Mississippi River.
The Louisiana Purchase (1803) was a foreign policy success for the United States primarily because it ended French control of the Mississippi River.
US President Thomas Jefferson wanted to purchase the Louisiana territory that belonged to the French. He sent James Madison to Paris in order to accelerate negotiations and to get a good deal for the immense portion of land. The US accepted to pay France almost $11,250,000 for 828,000 square miles of territory. With the purchase, the US ended French control of the Mississippi River.
The other options of this question were A) it secured full control of Florida from Spain. C) ended the British occupation of forts on American soil. D) eliminate Russian influence in North America.