If no candidate receives a majority of the Electoral College votes in a US Presidential election, the states' delegations to the House of Representatives select the president. Each state's delegation receives one vote. The House must select from the top three Electoral College vote getters (i.e. the three candidates with the highest Electoral College vote totals), and the winner must receive the majority of votes.
A minimum 2/3rds quorum (i.e. 2/3rds of the state's delegations must be present, and the winner must get a simple majority of that quorum). Only state delegations can vote in such a tie-breaker (e.g. the District of Columbia's Electoral representatives are excluded, and D.C. does not get a vote). Voting rounds continue until there is a winner.
So it is
B. House of Representatives.
