Which of the following Federal Reserve monetary policy tools has been most recently added?

Reserve requirements refer to the regulations set by the Federal Reserve that dictate the minimum reserves each bank must hold against deposits. While these requirements have been adjusted over time, they were not a new addition to the Federal Reserve's toolkit, as they have been in place for decades.

The discount window is a longstanding monetary policy tool that allows banks to borrow funds from the Federal Reserve, typically at a higher interest rate than the market. This tool has existed for many years and has not been recently added or modified; it remains a vital source of liquidity for banks.

Open market operations involve the buying and selling of government securities by the Federal Reserve to influence the supply of money in the economy. This method has been a foundational monetary policy tool for the Fed since the early 20th century and has not been newly introduced or amended in recent years.