Impeachment of a federal official is

the removal of the official from his or her office while waiting for an investigation and court trial

the removal of the official from his or her office because of widespread petitions from the people

when charges are brought against the official for criminal activity and a substitute must take office

when charges are brought against the official for evidence of treason, bribery, or other serious crimes

The correct answer here is: When charges are brought against the official for evidence of treason, bribery, or other serious crimes.

Explanation:

The explanation comes from the definition and purpose of an impeachment. Literally, an impeachment is the process by which the legislative branch sets charges against a government official for serious crimes, or misdemeanors, carried out by said official. This process is not the same as a conviction, but rather, it initiates an investigation against the official being accused. Because of this, officials are not removed from their posts, and they will not be removed, or replaced, unless a conviction is obtained. Because the repercussions are so severe when an impeachment happens, and because impeachment literally reverses the process by which an offical reached office, the process is reserved only when severe criminal activities or misdemeanors are reported; like bribery, treason, or other serious crimes.