The Rank and File Company is considering a stock issue to raise $50 million. An underwriter offers

Question:

The Rank and File Company is considering a stock issue to raise $50 million. An underwriter offers to guarantee the success of the issue by buying any unwanted stock at the $25 issue price. The underwriter's fee is $2 million.
a. What kind of option does Rank and File acquire if it accepts the underwriter's offer?
b. What determines the value of the option?