Who officiates when a president is tried for impeachment?

The Chief Justice officiates when a president is tried for impeachment as outlined in the U.S. Constitution. This unique responsibility is stated in Article I, Section 3, which specifies that in cases of impeachment of the President, the Chief Justice shall preside over the proceedings in the Senate. This ensures that the trial is conducted with a level of judicial oversight, given the gravity of the impeachment process, which may lead to the removal of the President from office.

The role of the Chief Justice is particularly important because it upholds the principle of impartiality and the rule of law in what is a highly political process. By having the Chief Justice involved, it reinforces the judicial branch's role in overseeing a constitutional process, thus preventing any potential biases from other political figures who might have vested interests in the outcome.

The other positions listed do not have a role in officiating the impeachment trial of the president as stipulated by the Constitution, which is why they do not align with the correct answer.