Final answer:
The statement that best describes the problems America had trading with Spain is that Spain blocked U.S. ships from traveling the Mississippi River, impacting southern trade. The correct option is C.
Step-by-step explanation:
The problems America had trading with Spain were best described by the fact that Spain blocked U.S. ships from traveling the Mississippi River. This move by the Spanish government significantly affected American southern merchants, who relied on the Mississippi River to conduct their international trade through New Orleans.
Disputes over borders and trade policies, as well as the lack of a central power to imposition tariffs or regulate trade further complicated the situation.