The piece of dialogue that would best help develop Casey's character for readers 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 reflects her lighthearted nature and tendency to joke, as it includes a grin and an enthusiastic expression of her discovery. It showcases her playful character while also addressing the serious concern about the mouse in a humorous way.