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 dialogue showcases Casey's lighthearted nature, as indicated by her grin even while discussing a potentially serious issue. It reflects her tendency to approach situations with a sense of humor, even when she is trying to convey that something is genuinely happening. This aligns well with her character description.