Which of the following government agencies determines how much money is in the economy?

Federal Reserve

Explanation:

You forgot to put the alternatives, but I absolutely know the answer is the Federal Reserve.

The Federal Reserve is the government agency responsible for conducting monetary policy in the US. One of the mechanisms of monetary policy is precisely the decision to inject or withdraw money from the economy. This is because the Federal Reverve is responsible for controlling inflation, which is a monetary phenomenon caused by excess currency in circulation. Thus, to contain inflation, the Fed uses a contractionary monetary policy (withdrawing money from the economy). In contrast, to expand economic activity in times of low inflation, the Fed pours money into the economy. All this is done by buying / selling public titles.