The correct answer is D: the way the main problem is introduced.
In "Give up Purple, Give up Black," the main problem revolves around the twins' concern about their parents' relationship and the emotional turmoil they experience due to their actions and choices. The problem is introduced through their dialogue and interactions with each other, leading to an emotional climax when they confront their parents.
In contrast, in "Party Problem," the main problem relating to the unexpected party for Darla is introduced through a series of text messages and a misunderstanding. This introduces a social dilemma that hinges on communication technology, which shapes the plot's direction and affects Darla's immediate reactions and concerns.
Thus, the difference in structure influences how each story's main problem is presented and developed.