The correct options for why Congress decided the election of 1800 are:
A. Based on the outcome, it was mandated by the U.S. Constitution.
C. no candidate won the electoral vote.
In the election of 1800, both Thomas Jefferson and Aaron Burr received the same number of electoral votes, which led to the decision being made by the House of Representatives, as mandated by the Constitution. There were not enough votes separating the candidates to declare a winner outright from the electoral vote, making it necessary for Congress to intervene.
Option B is incorrect because the popular vote was not a tie, and Option D is also incorrect because enough citizens had voted to determine the electoral outcome, but the result was a tie in the electoral college.