A) Overriding a presidential reversal or veto.
The Congress' right to override a presidential reversal or veto, is part of the system of checks and balances established in the U.S Constitution.
In the United States, once a version of a bill is passed in Congress, by a majority of the vote, the bill goes to the President who decides whether to sign it into law or reject it, that is to say, use the veto.
If the President uses his power to veto the bill, the Congress can override that presidential reversal by 2/3 vote of both houses of the Congress.
