C. The spelling bee contender flashed a self-possessed smile as the final word was announced.
Explanation:
You have not pointed out which words are underlined, but either way, we can see that sentence C is the only one that contains a word that is used incorrectly. This word is <em>self-possessed</em>. A smile can't be self-possessed. This adjective is used to describe a person - someone who is self-possessed is calm, confident, and in control of their emotions.
