Final answer:
King says that America has defaulted to suggest that it has failed to fulfill its promises and obligations, and this word choice is effective because it implies a serious breach of responsibility and trust similar to a financial default. The correct answer is (b).
Step-by-step explanation:
The student's question about why King says that America has defaulted and what makes this word choice effective can be answered by understanding the context within which it was used.
The correct answer is b. King uses "defaulted" to suggest that America has failed to fulfill its promises and obligations. This term is generally associated with financial settings and is particularly impactful because it conjures images of broken promises and the consequences that follow.
Defaulting on a loan implies serious irresponsibility and damage to credibility, which can have ripple effects throughout various sectors of society and economy. By using this term, King is highlighting the gravity of America's failure to live up to the ideals of freedom and equality for all its citizens.