Final answer:
The Electoral College system enables a candidate to lose the popular vote but still win the presidency through a majority of electoral votes due to the 'winner-take-all' approach used by most states.
Step-by-step explanation:
The factor that makes it possible for a presidential candidate to lose the popular vote but still become President of the United States is a) The Electoral College system. The Electoral College consists of electors equal to the combined total of a state's Senators and Representatives, plus three electors from the District of Columbia. Candidates win electoral votes by securing the majority popular vote in each state. Due to the 'winner-take-all' approach, except in Maine and Nebraska, it is possible for a candidate to win the presidency even if they lose the nationwide popular vote, as long as they win enough states to gain a majority of the electoral votes. This has happened in several elections, such as in 2000 and 2016, and has been a subject of debate and calls for reform.