The U.S. government ensures stability in the market economy primarily through the control of interest rates. This is typically managed by the Federal Reserve, which adjusts rates to influence economic activity, inflation, and employment levels. By raising or lowering interest rates, the government can encourage or discourage borrowing and spending, which helps stabilize the economy.
The other options provided do not accurately reflect how the government maintains market stability. For instance, the government does not encourage the creation of monopolies as they can lead to market inefficiencies and reduced competition. While states do collect taxes, the federal government typically sets parameters for taxation rather than preventing it altogether. Lastly, while the U.S. government may engage in international trade and investment, purchasing companies in other countries is not a primary tool for ensuring domestic market stability.