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 showcases her lighthearted nature as she grins while talking about the mouse situation, indicating that she maintains a playful attitude even when discussing something potentially alarming. It also reflects her humor and enthusiasm, allowing readers to see her personality come to life in this context.