A.  from out of nowhere – literally out of the blue
B.  from reactions to people, situations and things
C.  from thoughts about people situations and things

If you picked C you are correct.  Emotions do not come from out of the blue nor do they come from people, situations and things – they come from your thoughts about people, situations and things.

Case in Point:   You are driving home from the office and feeling frustrated about a meeting earlier that day.  You may attribute your frustration to the meeting but the truth is, the meeting is over and no longer exists.  What you are experiencing is your reaction to your thinking about the meeting.  You are literally feeling your own thoughts in the moment you are thinking them.  You are in your car driving home and no longer in the meeting.  You could be listening to nice music or noticing with interest what is around you.