Final answer:
The authors mean that the heart of the American dream lies in the belief of a person's ability to achieve upward economic and social mobility through hard work, ambition, and dedication.
Step-by-step explanation:
When authors mention that mobility is the promise that lies at the heart of the American dream, they refer to the belief that anyone, regardless of where they come from or what their social status is, can achieve success and upward economic mobility through hard work, dedication, and following societal rules. This idea encapsulates option A: A sense of ambition and aspiration drives the American dream. This concept was famously described by James Truslow Adams in 1931 and implies that life should be better and richer for everyone, with opportunities available according to each person's ability or achievement, irrespective of their origin or social class.
The American Dream emphasizes the importance of opportunity and the belief that hard work and ambition can lead to success, which is a stance that has historically attracted many immigrants to the United States with the hope of improving their livelihoods. The attainability of the American Dream has been questioned, particularly during and following the recession beginning in 2008, which had significant impacts on economic stability and job availability, testing the resilience and adaptability of the American economy and people's faith in the dream.