Final answer:
Merlin's poem in 'Magic Tree House: Summer of the Sea Serpent' directs Jack and Annie to start their quest by unlocking the sea serpent's treasure chest. The correct answer is option B.
Step-by-step explanation:
In the book 'Magic Tree House: Summer of the Sea Serpent', Merlin's poem guides Jack and Annie to begin their quest by unlocking the sea serpent's treasure chest. This starting point sets them on an adventurous path where they encounter mythical beings and face various challenges. The quest is intertwined with elements of magic and mystery, typically found within the series, which spark the imagination and provoke curiosity about the legendary themes.