As chief diplomat, the President does all of the following except what?A)negotiates treaties, B)deploys troops abroad, C)entertains foreign heads...

I disagree with your hint.  The answer to this question
clearly is B.  The duty of deploying troops is not part of the president's role as chief
diplomat.  Instead, deploying troops to places like Afghanistan and Iraq is part of the
presidents role as the commander-in-chief of the armed
forces.

Diplomacy has to do with conducting relations with
foreign countries.  All of the other options that you have given are clearly part of
normal diplomatic relations.  Entertaining heads of state and negotiating treaties are
clearly involved in maintaining relations with other countries.  Appointing US
ambassadors is part of this as well because those ambassadors are the president's
diplomatic staff.

Sending troops to war, by contrast, is
not something that is typically seen as diplomacy.  Instead, it is part of what the
president does in commanding the armed forces.