What best defines a broad market index?

A broad market index represents the performance of an entire asset class by aggregating the performance of a wide range of securities within that asset class. This index aims to provide a comprehensive view of the overall market's movements, enabling investors to assess how an entire market segment is performing rather than focusing on specific sectors or individual stocks. It includes a diverse array of stocks, often spanning various industries, which helps to mitigate risks associated with individual stock price changes.

In comparison, other options are more specialized in nature. Measuring the performance of selected individual stocks would not capture the comprehensive market movements that a broad market index aims to provide. Tracking the performance of a specific industry sector limits the scope to a narrower range of securities, rather than representing the entire market. Lastly, indicating the price change of commodities pertains to a different asset class entirely, focusing on physical goods rather than equities. Therefore, option B accurately reflects the definition of a broad market index.