The piece of dialogue that would best help develop the character of Casey 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 showcases Casey's lightheartedness and tendency to joke around, as indicated in the character description. The use of "oh!" and the grin convey her playful nature while still conveying the seriousness of the mouse situation, highlighting both her lighthearted demeanor and her urgency in informing her mother.