The piece of dialogue that would best help develop Casey's character as lighthearted and humorous is:
“Lots of reasons. There were nibbled food boxes, the scratching sounds in the wall—oh!” Casey grinned. “And I saw a mouse in the kitchen this morning.”
This response captures her playful nature with the mention of "nibbled" food boxes and her grin, which suggests that she maintains her lighthearted demeanor even in a potentially serious situation. The humor and playfulness shine through, aligning with the character description.