The correct answer is b. The audience applauded the ambassador, who spoke for one-half hour. A relative clause is a part of the sentence that depends on something else. It cannot stand alone. For example, while "The audience applauded the ambassador" can work as its own sentence, "who spoke for one-half hour" cannot. It is dependent on the first part of the sentence. Additionally, relative clauses will describe what it is dependent on. For example, "who spoke for one-half hour" is directly describing "the ambassador."
