B. Both were used to give legitimacy to the form of government being established.
The sort of justification or legitimacy is very different in the two approaches, however. The US Constitution begins with the words, "We, the people," because the people are the ones establishing the government and constituting its legitmacy according to the laws set forth in the Constitution. The idea of "The Mandate of Heaven" was more like the old European idea of divine right monarchy--that kings are appointed by God above. According to the Mandate of Heaven, it is Heaven that grants an emperor the right to rule, and only one emperor can rule at a time because there is only one Heaven. An emperor's virtue is the basis for his right to rule, so if he fails to rule virtuously, Heaven will remove his right to rule.
