Answer:
The sentence that contains a compound subject is B. Their friends and relatives from Austria visit often.
Step-by-step explanation:
The subject of a sentence is the person, thing or idea that carries out the action defined in the predicate. When the sentence has two or more subjects, that subject is called compound subject. In a compound subject, the individual subjects are joined by a coordinating conjunction, such as and, or, etc. In this case, the individual subjects that form the compound one are their friends and relatives (from Austria.)