D. sales and investment spending by businesses.
The national income and product accounts (NIPA) is a system of accounts used to track economic activity in a country. It includes a variety of measures, such as Gross Domestic Product (GDP), personal income, and corporate profits. One of the primary functions of the NIPA is to keep track of sales and investment spending by businesses, which are important components of GDP. This information is used by policymakers, economists, and businesses to analyze and make decisions about the economy. The NIPA does not track worker productivity, consumer debt, or unemployment rates directly, although these factors can indirectly affect the measures tracked by the accounts.