The correct answer is b) to limit the power of the ruler
The Mandate of Heaven has four basic principles, the right to rule is granted by heaven but this did not mean the ruler could do whatever he wanted - as an example of European rulers - this means that the right to rule is based on the virtue of the ruler, if he does not meet the expectations he does not deserve the right to rule.
At that time Chinese Society had a strict moral code, the ruler had to follow it to deserve power. There are some episodes when there were natural disasters happened and people interpreted it as a sign that the ruler was unjust and needed a replacement.
