Monetary policy is defined as the

buying and selling of currency in foreign exchange markets.
interaction of buyers and sellers in the market place.
decisions of the federal reserve system that determine the money supply.
taxing and spending decisions of the united states government

Decisions of the Federal Reserve System that determine the money supply.

Explanation:

Monetary policy is known to be an economic policy used by the Federal Reserve System or Central Bank in controling  the supply of money in the economy of a country. The supply of money in the economy can be inform of cash, checks, credits and so on. They affect the supply of money by reducing liquidity in order to stop inflation which is a general rise in the price of goods and services and by so doing, prices become stable. Thus, they also  increase liquidity which in turn has a positive impact on the economic growth of the country.

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