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 line captures Casey's lighthearted nature by including her grin and the playful way she presents her evidence for the mouse, even while discussing a potentially serious issue. It showcases her tendency to find humor in situations while still being earnest about the problem at hand.