Correct answer: a. designed to select the president.
As noted on the Electoral College page at the National Archives site, "the founding fathers established it in the Constitution as a compromise between election of the President by a vote in Congress and election of the President by a popular vote of qualified citizens."
There are many checks and balances in the design of US government. The Electoral College is a check against what the founders thought of as possible "tyranny of the majority," if a candidate with a great number of popular votes was seen by the selected electors as dangerous to the republic.