There are five interludes in this book, and the fire at the Black Spot is discussed in the second interlude. This is answer A, so that is the correct answer. For those who haven’t read the book, this is a major spoiler alert, as I’m going to go into what the interludes do and why they were included in the book.
The interludes serve as a journal from the point of view of the character Mike. Interlude 1 is kind of an introduction where we meet Mike. The next three (so, interludes two through four) are stories that Mike is writing down in his journal to keep them for posterity as far as I can tell. Interlude 5 concludes the stories from Mike. They were included to give history behind the things that happen in It!, and are a hefty read.