The facial-feedback hypothesis tells us that

a. facial expressions are of little importance in communicating.
b. we can tell how a person is feeling by his or her facial expression.
c. smiling makes a person feel better.
d. smiling is often a cover-up for emotional disorders.

Facial-feedback hypothesis is a psychological concept which proposes that an individual’s external facial movement can influence a person’s inner emotional experience.  The idea was first proposed by Charles Darwin and later by the psychologist William James.  

Thus, from the given options, the best answer would be (C) smiling makes a person feel better.