The element of the setting that also becomes a main character in "The Adventures of Huckleberry Finn" is:
D. the Mississippi River
Explanation:
In Mark Twain's "The Adventures of Huckleberry Finn," the Mississippi River is as much a character as Huck and Jim themselves. The river is what takes Huck and Jim places, symbolizing freedom. But, besides that, the river, just like another character, is able to get them in and out of trouble, serving as a plot device. In other words, the river helps move the plot forward. The adventures mentioned in the book's title only happen because of the river.
