Correct answer: D.
First, ask yourself what kind of person reveals secrets about others, especially for personal gain. Certainly it will be a negative word, so we could look for something that means "disloyal" or "untrustworthy."
The correct answer choice is (D) since "perfidious" means "willing to betray trust," something the personal assistant was willing to do. None of the other answer choices could be correct because they do not match that definition. (A) "diffident" means "timid"; someone (B) "estimable" is of high honor and reputation; (C) "caustic" is "acidic" (certainly negative but doesn't make sense in the sentence); (E) "erudite" means "scholarly."
