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 option showcases Casey's lightheartedness and humor with her grin and the way she lists the reasons for believing there’s a mouse. It highlights her personality as someone who enjoys joking around while still showing her genuine concern about the presence of a mouse in a fun and engaging way.