Read the sentence. The flag of the United States, which is often called the Stars and Stripes, originated during the Revolutionary War. Which statement about the underlined phrase is true? The phrase is a relative pronoun. The phrase is an essential adjective clause. The phrase is a nonessential adjective clause. The phrase is connected to the sentence with a relative adverb.

Based on your options, I can assume the underlined phrase is which is often called the Stars and Stripes, even though you didn't actually underline anything. Anyway, the correct answer here is that the phrase is a nonessential adjective clause.

Nonessential clauses, as the name itself says, are not that relevant for the meaning of the entire sentence. If you omit that clause, the meaning won't change, whereas an essential clause cannot be omitted. Additionally, nonessential clauses are separated from the rest of the sentence by commas, which is the case here.