Final answer:
Cal and Toby read Wayne's last letter to Cal in their treehouse, a location that is emotionally significant to the characters in the narrative. Option d is the correct answer.
Step-by-step explanation:
The correct answer to the question 'Where do Cal and Toby read Wayne's last letter to Cal?' is that they read the letter in their treehouse. This scenario occurs in a book, and understanding the context helps to comprehend the significance of the location chosen for reading the letter.
The treehouse likely represents a place of personal and emotional significance to the characters, making it a fitting location for such an important and intimate moment in the story.
SO Option d is the correct answer.