Final answer:
Quote B from Romeo and Juliet is the best support for the theme of the feud between the Montagues and Capulets leading to the lovers' tragic end, as it explicitly links their death-marked love with the continuance of their parents' rage.
Step-by-step explanation:
The quote that best supports the theme of how the feud between the Montagues and Capulets leads to the tragic end of Romeo and Juliet, from the given options, is B. “The fearful passage of their death-mark’d love, / And the continuance of their parents’ rage, / Which, but their children’s end, nought could remove” (Lines 9-11). This quote encapsulates the essence of the tragic outcome being a direct result of the familial enmity and highlights that nothing apart from the death of their children could end the parents' hatred. This selection closely aligns with the precedent set in the prologue of Shakespeare's Romeo and Juliet, which mentions the feud and its fatal consequences.
While the other quotes are relevant to the play, they do not as directly support the strife between the two families as the reason for the tragedy. This makes quote B the strongest choice for supporting the given theme.