The piece of dialogue that would best help develop Casey's character 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 her lightheartedness and humorous nature, especially with her grin indicating she's both playful and pleased to share her findings, reinforcing her character as someone who enjoys bringing a sense of fun to potentially serious situations.