Answer:
C. Two-thirds vote
Explanation:
To help with checks and balances, the consitution allows congress to override a presidential veto with a two-thirds vote; also known as a constitutional majority.
Intial Vote
When Congress first votes on a normal bill (not a consitutional change) all that is required is a simple majority. A simple majority means more than 50 percent.
Presidential Veto
Just because something is passed by Congress does not mean that it will be a law. The President still has to sign the bill into law. However, the President has the option to veto the bill.
Overriding a Veto
If a bill is overridden, Congress can move to override the veto. If they want to override the veto, there must be a 2/3's majority that agrees to pass the bill again.
Additionally, when overriding a veto any votes that are not affirmative, this includes abstentions and absent voters, are considered opposing votes. For example, if 2 congressmen did not show up to vote, then their votes would automatically be counted against the bill.