Answer: The third one, She had puppies? How exciting! How many?.
Explanation: When you finish writing a sentence, your supposed to add a capital letter to start the next sentence, because its proper grammar.
Example: I just ate dinner. It was so yummy!
Opposite: I just ate dinner. it was so yummy!
So it would be the third one (C), because when the writer finished each sentence, they added a capital letter. "→She had puppies? →How exciting! →How many?"
Hope this helps! :)