The correct answer is: "the interest rates".
Central banks, such as the Federal Reserve, are in charge of designing and applying the monetary policy, which consists on influencing the amount of money in circulation in the economy and the interest rate levels, through mechanisms such as the open market operations.
These two factors allow to determine the availability of credit in the economy and its investment levels, which in turn determine GDP figures and economic growth levels. The main objectives of the monetary policy are to ensure high economic growth, high employment levels and price stability (low inflation).
