Someone who is a "good sport"
a. argues against bad calls
b. celebrates excessively
c. congratulates opponents
d. shows off during games

C. congratulates opponents

Explanation:

A person who is a good sport is someone who does not take competition or winning and losing too seriously. This type of person is generally kind to his opponents, even to the point of congratulating them. Moreover, they generally value the spirit of the game highly and accept defeat gracefully. Finally, these people are more likely to value the fun aspects of the game, as opposed to simply valuing victory.