Which is the best example of checks and balances?
a. congress may override a president's vetoing of a bill
b. the supreme court bases its judgments on constitutional law
c. the president works to ensure that existing laws are enforced
d. the constitution may be amended to fit changing circumstances

A. Congress may override a president's veto. 
The idea behind checks and balances is to spread the power evenly between each government branch, (Separation of powers). While the president can veto a Congressional bill thus exercising a separation of power he may be paradoxically overrided if 2/3 of both the House and the Senate vote in favour of doing so.