Final answer:
Sera leads Mr. Thorne to the mountain lion's den to show him where it lives.
Step-by-step explanation:
Sera leads Mr. Thorne to the mountain lion's den because she wants to show him where the mountain lion lives. This is likely because Sera is familiar with the area and knows where the den is located.
By leading Mr. Thorne to the den, she is able to provide him with first-hand knowledge of the mountain lion's habitat.