When characters say something that's the opposite of what they mean, they're using

A. conflict.

B. symbolism.

C. sarcasm.

D. irony. (Correct Answer)

The correct answer is D. irony.

Irony is a figure of speech used when you want to state something completely different than what you actually mean. For example, you can ironically say that money is not important (even though you really think it is). Even though sarcasm and irony are sometimes similar, sarcasm is mocking, but not necessarily using opposite meanings.

The correct answer is D. irony