The line that best illustrates Lewis Carroll's use of opposites in "Through the Looking-Glass" is:
“When you say ‘hill,’” the Queen interrupted, “I could show you hills, in comparison with which you'd call that a valley.”
This line embodies the idea of opposites by contrasting the concept of "hill" with the idea of "valley," suggesting that there are varying degrees of height and perspective. It emphasizes the theme of relativity, which is prevalent throughout the novel, where situations, characters, and concepts are often viewed differently depending on one's perspective. This interplay of contrasting ideas enhances the surreal and whimsical nature of Carroll's world.