Which of the Following Best Defines an Exercising the Option

Which of the following best defines an exercising the option?

A) A bond that can be exchanged for a fixed number of shares of stock for a specified amount of time.
B) An option granted to an employee by a company giving the employee the right to buy shares of stock in the company at a fixed price for a fixed time.
C) An option that can be exercised only on the expiration date.
D) The act of buying or selling the underlying asset via the option contract.
E) The last day on which an option can be exercised.