Answer:
I believe the complexity in the vocabulary of the last half of the poem shows:
D. the narrator's maturing thoughts.
Explanation:
In just a few lines, the narrator goes from childish thoughts to more mature ones. His/her tone and vocabulary are a representation of that. At the beginning of the poem, the language is more direct, less adorned. By the second half, it sounds more poetic, even. "Their embrace reeks of dreams," for instance, is probably the most beautiful and mature line of the poem. Not only it conveys the narrator's perception of his/her parents' expectations in a lyrical way, but it also shows that the narrator is mature enough to take such expectations into consideration. So much so that he/she says, "I will walk first and perhaps trace the path." Even without much certainty, there is love, consideration, empathy, while at the beginning there seemed to be only fear and doubt.