I would pick the idea in C, except for the fact that the raft going south is filled with irony. Jim is not heading to where he could claim freedom: he's heading in the wrong direction. He should be heading North, not South. South is the location of slavery. If C mentioned that fact, I would pick it.
Instead you have to pick a more mundane answer.
I suppose I would take D, but it is not nearly as important as C.
The setting is a raft traveling down the Mississippi River, which allows Huck to experience a degree of freedom that helps him understand how important freedom is for Jim.
I believe the answer is heroic couplets, which is a traditional form for English poetry, commonly used in epic and narrative poetry, and consisting of a rhyming pair of lines in iambic pentameter.
