Which of the following samples is likely to be the most representative of the population of all registered voters in the United States? 

a. A sample of 200 people at a Denver shopping mall. 

b. A sample of 200 of your friends and their friends. 

c. A sample of 200 people, chosen by dialing telephone numbers at random. 

d. A sample of 200 people selected at random from all students at the University of Nebraska.