Final answer:
Lady Capulet likens Paris to an unfinished book, suggesting that he is appealing both internally and externally, and could be completed with Juliet.
Step-by-step explanation:
The statement that BEST summarizes Lady Capulet's extended metaphor is that she compares Paris to a book that is beautiful on both the inside and outside. In 'Romeo and Juliet', Lady Capulet describes Paris as a prized volume still lacking a cover. Juliet is encouraged to read Paris like a book and to find what is written in the margin. Lady Capulet suggests that Paris's story can be complete if Juliet is the cover, thereby making a union that is as lovely inwardly as it’s outwardly.