The correct answer is C. It uses an informal expression.
Explanation
Credibility is a characteristic that refers to the ability to be believed. In general, the media must have credibility because they use formal language, and support the information in reliable sources, contributing to credibility. When a text, speech, or person has credibility it generates trust in others. According to the above, the headline "President's Speech Is a Total Bummer!" does not have credibility because it uses an informal word as "Bummer", causing little credibility in the receiver of the message as this gives an impression this newspaper is not serious or expert in the topic. Therefore, the correct answer is C. It uses an informal expression.