Answer:
D. Statistics showing the difference in pounds of trash between the two types of highways.
Step-by-step explanation:
In the given text, the author describes how trashes and garbage along the highways have increased with the increase in the number of people traveling. For this reason, he proposes that people "adopt-a-highway", a scheme where people can adopt a highway and make it their responsibility to keep it clean. This will not only ensure a clean highway but also saves the state's money which can be used for other more important uses.
And in support of his argument, the author also provides certain statistical evidence. he provides the statistics on how different the pre and post "adopt-a-highway" scenario looks like. He mentions that "The South Carolina Litter Patrol statistics [. . . prove] that the programs work". And in providing the statistics, he is making sure that his argument has a supporting detail.
Thus, the correct answer is option D.