An example of the system of checks and balances is B: Congress can override the president's veto.
This demonstrates how one branch of government (the legislative branch, represented by Congress) can check the power of another branch (the executive branch, represented by the president) by overturning a presidential veto of legislation, requiring a supermajority vote in both the House and Senate. This ensures that no single branch can dominate the legislative process without the other branches having a say.
However, please note that the specific option you mentioned ("Congress can override the president vote") seems slightly unclear, as it would typically be phrased as "Congress can override the president's veto."