Answer: C. increase the available money supply
Step-by-step explanation:
The supply of money is the total amount of money that a particular economy has at a particular point in time.
The Federal Reserve purchasing large amounts of treasury securities is an expansionary policy which means that it's a policy that is used to increase the money that's available in the economy.
When the government buys securities, it leads to more money on circulation. Likewise, if the federal government wants to reduce money supply, it sells securities to the people.
Therefore, the correct option is C.