The correct answer is "a specific character’s costume required the occasional use of fire onstage, so the costume designer and special effects crew would have to work together to design that costume. "
The character that requires the work of both the costume designer and the special effects crew is Lumière, the candlestick. Both parties need to work together to ensure the costume is made of safe, fire-resistant fabrics, and that the special effects does not interfer with the costume itself.