Imagine that you are writing a report on the people who most influenced a president during the administration. which advisor would be the best source on a president’s personal thoughts? the national security advisor the secretary of state the attorney general the chief of staff the secretary of treasury

The Chief of Staff.

All of the advisers listed in your question have important roles to play, but the White House Chief of Staff is continually in contact with the President, and so would be the one more likely to have insight into the personal thoughts of the president.  The Chief of Staff controls the flow of other persons into the Oval Office for time with the President.  The Chief of Staff also serves as an adviser to the President and a negotiator for him with other members of the government.  The Chief of Staff is the person most often at the President's side.  Also, whereas Cabinet officials and the National Security Adviser advise the President on issues specific to their appointed roles, the Chief of Staff is involved in the President's planning and communication in regard to all issues of the administration.