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 showcases Casey's lightheartedness and tendency to joke around, as implied by her grin while discussing a potentially serious issue. It reflects her character's playful nature while still addressing her concern about the mouse.