Final answer:
A national decrease in consumer spending on entertainment is the best example of unsystematic risk, as it affects specific companies or industries rather than the entire market.
Step-by-step explanation:
The best example of unsystematic risk from the options listed is 'a national decrease in consumer spending on entertainment'. Unsystematic risk refers to the risk that affects a specific company or industry as opposed to the entire market or economy. The other options listed, such as an across-the-board increase in income taxes or a decrease in the national level of inflation, are examples of systematic risks which impact the entire market or economy.
Examples of unsystematic risk include unique events like a company losing a major lawsuit, a sudden strike by workers, or a scandal involving company executives. These events are not correlated with the market's overall movements, and therefore, they only impact the particular companies or industries involved. In contrast, systematic risk affects all securities in the market including broad factors like changes in government policy, economic recessions, and natural disasters.