The correct answer is: C: The sentence has a compound subject. Subject in sentence is<span> 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.</span>
I would say C but if I am wrong please let me know.
Hope this helps
