D. They formed trading posts along the Mississippi River.
The French were the one European group not really focused on converting the local Native Americans - A. is more accurate for the Spanish. B. Is incorrect as well, as Spain was the first to discover and explore Florida and the Gulf Coast. They had no need of slaves, because they were mainly focused on hunting and trade, not agriculture, so they did not introduce slavery to the area. However, they did form trading posts along the Mississippi to gain valuables from the Native Americans there. They sold beaver pelts, and the fur of other hunted animals.
A parallel structure is a sentence that repeats similar word structures twice.
In this case, the correct answer is sentence 3.
For example: She not only wanted a dog, but also wanted a cat.
