I would say the correct answer is C) by revealing shocking information. It's not that the information is withheld much - first, we suppose that the person who laughed would be Fortunato; second, this assumption is confirmed by the end of the sentence. The "low laugh" is a shocking information in itself, as it is definitely not expected in a moment when we already think that Fortunato is dead (and even if he isn't, he definitely can't have a reason to laugh).
