Final answer:
The correct answer to the student's question is the stock market, specifically referring to the Dow Jones Industrial Average surpassing the 10,000 mark on March 29, 1999, which doubled the average price of stocks in the index and benefited brokers invested in the market.
Step-by-step explanation:
The subject in question pertains to the event that occurred on March 29, 1999, when a specific index broke the 10,000 mark for the first time. Considering the information provided and the historical context of stock market trends, it is evident that the correct answer is a) Stock market. The Dow Jones Industrial Average reaching the 10,000 milestone was a significant event, reflecting a period where stock prices saw a dramatic rise from the late 1980s up to around 2000, before experiencing volatility in the following years.
Figure 17.6 indicates that when the Dow Jones average rises from 5,000 to 10,000, the average price of the stocks in that index has roughly doubled, which aligns with the mentioned event in question. This milestone made some brokers richer as they had various investments tied up in the financial markets, and the high index meant increased wealth for those invested in the stock market.