The correct answer is A. over.
A private secretary is a person who is employed to assist a high-ranking person, such as a business executive or a politician. The secretary’s duties typically include answering the boss’s phone, scheduling appointments, and taking dictation. A private secretary does not have authority over his or her boss. In fact, the opposite is true: the boss has authority over the secretary.
The other options are incorrect because they do not accurately reflect the relationship between a private secretary and his or her boss. Option B, upon, suggests that the secretary is responsible for or accountable to the boss. Option C, on, suggests that the secretary is in charge of or has control over the boss. Option D, for, suggests that the secretary is working on behalf of the boss.
In conclusion, the correct answer to the question “A private secretary cannot have authority . . . . . . . . his boss” is A. over.