Which of the Following Illustrates the Best Example of "Flow

Which of the following illustrates the best example of "flow"?

A) Susan has just started her second day of swimming lessons and just swam two lengths of front stroke for her first time.
B) Collin's basketball coach told him to pay attention to where his feet were when taking a foul shot.
C) Juan typically finds that time flies when he is at work because he is enjoying himself.
D) Serge is a professional hockey player who spends extra time training at the gym with his friends.