The correct answer is: C: The sentence has a compound subject. Subject in sentence is the person or thing about whom the statement is made. Every verb in sentence must have at least one subject. But that doesn't mean that it is the only possibility: namely, the verb can have more than one subject, too. In such cases, we can say that a sentence has a compound subject. In this sentence: "Conrad and his sister will sing a duet in the concert.", compound subject is: "Conrad and his sister", since the statement is about the both of them.