Answer: A. Paris is an old city, but some is even more ancient.
Step-by-step explanation:
A is a compound sentence because the there are two complete sentences in the sentence. “Paris is an old city” is a complete sentence. “Some is even more ancient” is another full sentence. If there are two or more complete sentence separated by a conjunction, then It is a compound sentence. B is not a compound sentence, because there is only one complete sentence.