The sentence which shows that the narrator is conscientious about the environment is "I drove (guiltily, I'll admit!) up to the Colorado/Wyoming border..."
What does the sentence reveal?
Sentence D reveals that the narrator is conscientious about the environment because he admits to feel guilty about driving to the border. The reason for such guilt lies in the knowledge that driving creates pollution.
In other words, the narrator feels guilty for producing pollution when driving to the border, which shows his environmental awareness. With that in mind, option D is the correct answer.
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