How is a president chosen when none of the candidates receives a majority of the electoral votes? there is a national runoff election between the two candidates with the greatest number of electoral votes the us supreme court directly elects the president the election is ruled null and void and congress appoints a new president?

None of those choices are correctly stated, although the final option comes closest to stating it properly.

The government's own Electoral College site gives you a clear statement of what actually would happen.  If none of the candidates for President attains a majority of Electoral votes, the House of Representatives elects the President.  They would choose from the three candidates who received the most Electoral votes, and each state's delegation would get one vote in the process.  (So the state's representatives would need to determine which candidate their state was voting for.)  Then the Senate would elect the Vice President from the two Vice Presidential candidates with the most Electoral votes, with each Senator getting one vote in that process.

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