A taxpayer who has a Section 6651(a)(1) penalty has done which of the following? Select one: a. Failed to pay taxes due on their return, despite the fact that they filed the return on time. b. Failed to timely file their return. c. Failed to disclose an uncertain tax position and are due to pay a penalty as a consequence of substantially understating their liability. d. Failed to both file their return and pay the amount of any taxes due on their return.