Final answer:
Option (C), The decline in the natural rate of unemployment from 1980 to 2000 was primarily due to advances in technology and increased productivity, which improved labor market efficiencies and created new job opportunities.
Step-by-step explanation:
The bulk of the decline in the natural rate of unemployment from 1980 to 2000 is attributed to advances in technology and increased productivity. These factors played a key role in reshaping the labor market by improving efficiencies and enabling the creation of new kinds of jobs, which collectively reduced the natural unemployment rate.
Moreover, the influx of women into the workforce and new entrants to the labor force, who are typically counted as frictionally unemployed until they find a job, impacted the labor market dynamics but were not the primary cause of the decline in the natural rate during this period.