Answer:
A.) Madame Rammelk, who is quite a distinguished pianist, will be joining us for dinner.
Step-by-step explanation:
Sentence A provides the name, then adds an appositive clause (extra information to help understand context), and continues the sentence by describing the action of the subject.
Sentence B lacks the comma after "pianist" to mark the end of the appositive clause for clear separation.
Sentence C is ambiguous because it is difficult to tell if "who" is a pronoun that is referring back to Madame Rammelk or dinner, which can cause confusion.