Which of the Following Is the Best Example of a Phenomenon

Which of the following is the best example of a phenomenon?

A) Smiling people are generally evaluated more positively than non-smiling people.
B) Sam, a male waiter, drew a smiley face on a check and got a particularly poor tip.
C) Smiling people put us in a good mood, causing us to be more likely to help smiling people.
D) Smiling is a fundamental human behavior.