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.

The correct answer is a because it shows how one branch can directly override another. B is demonstrating how the Supreme Court works, c is an example of a job of the President, and d is not even about checks and balances. 

The  pan-african movement  was an "emotional, cultural, psychological and ideological  movement  that began among the  african  dispora in the western hemisphere, for a closer  purpose, so that  african  people could feel secure, attain political, economic as well as psychological power visa vis other races or world regions" .