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 lighthearted nature and her tendency to joke around, as indicated by the grin, while also providing legitimate reasons for her concern about the mouse. It balances her humorous personality with the seriousness of the situation, making her character more relatable and vivid for the readers.