Congress can override the president’s veto with

Congress can override the President’s veto with two-thirds vote in each Chamber

Explanation:

Direct veto is when a President has an objection to a bill or resolution.Then, the President will send the bill back to Congress explaining his objections. To override a presidential veto it is necessary to have  a two-thirds majority in both, the House of Representatives  and the Senate, as specified in Article 1, Section 7.  

i would say b. research typically uses a variety of sources