An open market purchase:

a. decreases the price of Treasury securities and also decreases their yield.

b. increases the price of Treasury securities and decreases their yield.

c. increases the price of Treasury securities and also increases their yield.

d. decreases the price of Treasury securities and increases their yield.

The answer is b. increases the price of Treasury securities and decreases their yield.

An open market purchase is when the Federal Reserve buys government securities from banks. This action pours cash into these financial institutions which allows them to increase the number of loans granted. Since consumers and businesses have more money to spend, the interest rates go down. This includes also a decrease in the yield of Treasury securities, and as the yield decreases, the price of these securities rises. This is due to an inverse relationship between a bond's price and yield to maturity.