Which of the following statements is an opinion?
a. the u. s. civil war was the worst our country has ever faced.
b. birds are vertebrates, with a backbone and skeleton.
c. margaret thatcher was the longest-serving british prime minister in the 20th century.

In the given choices above, the sentence that expresses an opinion is, The U.S Civil War was the worst our country has ever faced. The correct answer is option A. An opinion is a statement or judgment that is usually subjective, that is not based on any facts or knowledge. In other words, it is just a belief.