Aruler could lose the "mandate of heaven" if he or she
a. became immoral or unjust
b. did not convert to buddhism
c. did not pass the civil service exam
d. were not the first born son or daughter

A.became immoral or unjust

Explanation:

The "mandate of heaven" is a Chinese political and religious doctrine applying of the Emperor of China, or how they used to call the ruler of China: the "Son of Heaven" of their "Celestial Empire". Basically they believed that it was not required for a legitimate ruler to be of noble birth, but instead he would gain the right to rule based on the just and able performance of the rulers and their heirs, therefore if ever doing something immoral or unjust the ruler would lose the "mandate of heaven", the right to rule.

b: to eliminate competition from other farmers