D. The percentage of the labor force that is out of work and cross-country differences in average income.
Macroeconomists typically focus on the overall behavior of the economy, including aggregate measures such as unemployment rates, inflation, and national income.
The percentage of the labor force that is out of work and cross-country differences in average income are examples of macroeconomic topics.
In contrast, microeconomists study individual markets, individual consumer and producer behavior, and specific economic agents, making choices within those markets.
Options A, B, and C are more aligned with microeconomic topics, as they involve specific markets, prices, and individual consumer and producer decisions.