Which of the following benefits provided by an employer to its employees is taxable?

a. Employees of the ABC Department Store are allowed a 15 percent discount (which does not exceed the employer's gross profit percentage) on the retail price of all merchandise purchased from the store.

b. Undergraduate tuition is waived by the XYZ University for dependent children of employees who are admitted to the school.

c. The MNO Airline provides free standby flights to its employees.

d. None of the above benefits are taxable.