Whom may the president remove from office "at will" (i. e., for any reason whatsoever)? (select all that apply)?

The 1930’s and 1940’s witnessed a great growth in Federal bureaucracy. From this time, unelected officials from this bureaucracy have proved on occasions to be in competition with the president. Likewise, a president has to keep a close eye on the unelected Cabinet Secretaries who head each Executive department – virtually an impossible task due to the size of the Executive staff and the frequency with which these personnel have to be left to operate without someone metaphorically looking over their shoulders. Though those who head the Executive are hand picked by the president, he still needs as much skill in handling these people as he does with handling Congress.