The detail which best show that the narrator's activism has changed practices in the community is this: 'three weeks later, two businesses had signed on and two more said they do think about it. This option is the correct one because it shows that two businesses in the community has implemented the desired change already and two more businesses are on the verge of doing the same, which means the narrator's activism is already generating positive results in the community. <span />
It would be D the author wants to tell the reader what it means.
What is the story trying to show and what is it trying to teach?:)
Your birth date. So the answer is D.