Final answer:
The changing age structure and more efficient labor markets likely cause the decline in the natural rate of unemployment, as they lead to lower unemployment rates among prime-age and older workers and faster job matching. so, option D is the correct answer.
Step-by-step explanation:
The changing age structure of the US population and more efficient labor markets contribute to the decline in the natural rate of unemployment. As the age distribution shifts towards more prime-age and older workers, who typically have lower unemployment rates, the overall natural rate may decrease. Moreover, more efficient labor markets, aided by technology and better matching processes, can reduce the time it takes for workers to find new jobs, again contributing to a decline in the natural rate of unemployment rather than cyclical unemployment or the stagnation of real wages.
Both the changing age structure of the US population and more efficient labor markets can explain the decline in the natural rate of unemployment. In terms of the age structure, a society with a relatively high proportion of young workers tends to have a higher unemployment rate than a society with a higher proportion of older workers. This is because young workers are more likely to be trying out jobs and exploring different opportunities, leading to higher frictional unemployment. As for more efficient labor markets, advancements in technology and increased education and training can help match workers with suitable job opportunities, reducing structural unemployment.