Answer:
d. impeachment
Explanation:
Regarding federal pardons., Section 2 of Article II of the U.S. Constitution states the following:
The President (...) shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment.
When the President uses his or her power to pardon, they absolve of guilt a person for any legal offense and he or she can do it at any point after the commission of the crime. However, this power has limits as they can not grant pardons to impeached public officials, that is to say, to those that have been charged with misconduct before a court of law.