How was the 2012 U.S. election an example of an election that turned on voter tu

How was the 2012 U.S. election an example of an election that turned on voter turnout?

A) Republicans boosted the participation rates of their traditional voters, and pushed voter registration for those favorably disposed to the Republican Party, while the Democrats paid less attention to grass-roots work, concentrating instead on TV advertising.
B) Democrats boosted the participation rates of their traditional voters, and pushed voter registration for those favorably disposed to the Democratic Party, while the Republicans paid less attention to grass-roots work, concentrating instead on TV advertising.
C) Democrats boosted the participation rates of their traditional voters, despite Republicans pushing voter registration for those favorably disposed to their party, and concentrating on TV advertising.
D) Democrats boosted the participation rates of their traditional voters, and pushed voter registration for those favorably disposed to the Democratic Party, and despite Republicans paying attention to grass-roots work, and concentrating instead on TV advertising they still lost.

This is for my political science class.