Answer:
Correct answer is (C) They explain how people's understanding of Magna Carta's purpose has changed.
Step-by-step explanation:
C is correct because the main idea of the text is that Magna Carta is a symbol of liberty and even later influenced others.
A is not correct because this is not central idea, meaning that the idea is important than the existence of the original document.
Same goes with B and D, as this factual information are not that important for the main idea.