The correct answer is:
a When the same overt act was witnessed by two or more witnesses and/or a confession was given in public court.
Under the U.S. Constitution (Article III, Section 3), treason is defined as levying war against the United States or adhering to their enemies, giving them aid and comfort. A person can be convicted of treason only on the testimony of two witnesses to the same overt act or on confession in open court. This means option "a" aligns with these requirements.