An investor has established a short index put position and is assigned an exercise notice by his broker-dealer. Which of the following statements is true?
a) The investor is obligated to purchase the underlying asset at a specified price.
b) The investor is obligated to sell the underlying asset at a specified price.
c) The investor has the option to buy the underlying asset at a specified price.
d) The investor has the option to sell the underlying asset at a specified price.