Final answer:
A decrease in U.S. GDP (option D) is the best example of systematic risk, affecting the entire market or economy, unlike other options which represent unsystematic risks impacting specific sectors.
Step-by-step explanation:
Among the options provided, the best example of systematic risk is a decrease in the U.S. GDP. Systematic risk affects the entire market or economy, and not just a specific industry or company. This is contrasted with unsystematic risk, which impacts a particular company or industry. For instance, an increase in the productivity of auto manufacturers would be more related to unsystematic risk as it affects a specific sector.
Similarly, an increase in computer sales or a decrease in the production costs of cell phones also falls under unsystematic risk, impacting individual companies or sectors. Other examples of systematic risk include: a stock market collapse, rising inflation, a rise in the natural rate of unemployment, or a rise in oil prices, all of which have widespread impact on the economy.