B. An ancient Celtic fire festival.

Correct! The Celts observed four major fire festivals throughout the year: Halloween, or "Samhain," was one of these sacred festivals. Marking the end of harvest, it was a celebration of the beginning of the winter half of the year and thus an appropriate time to remember the dead, as well as a time for divination and reflection on the coming fortunes for the year. Later, the Christian observances All Saint's Day and All Soul's Day, were placed on the days immediately following Samhain.