The correct answer is:
The author's purpose for the allusion is to emphasize how seriously the relationship is doomed.
Romeo and Juliet was a tragic story. Two star-crossed lovers who could not be together, and have a sustaining relationship. Comparing Sasha and Karl to this really shows how these two face a similar proposition. Their relationship is doomed. They can't be together like Romeo and Juliet could not be together.
The Magna Carta was the document that reduced the power of the English Monarch.