S1. The beauty of India's democracy rests in the greatness of its Constitution. S2. The Constitution of India upholds the collective values of the country's civilizational ethos. The second statement:

The first statement claims that India's democracy's beauty lies in the greatness of its Constitution. The second statement explains why the Constitution is great — because it upholds the collective values of the country's civilizational ethos. Thus, the second provides a justification/reason for the claim made about the Constitution's greatness. It is not merely connecting to 'democracy' but specifically supporting the assertion about the Constitution. Hence (b) provides a reason for the assertion regarding the Constitution in the first is the most precise choice.