The sentence that refers to opinion is letter C: The United States government is the best in the world.
A fact is something – news, information, a talk, a seminar, etc., that is telling the truth and objective. This could be referenced by the people listening to it. An opinion is a declaration of the person’s thoughts and suggestions that is possibly subjective in nature. This cannot be referenced since it is declared by basing of emotions or one’s own perception on the information.