CART and the Indy Racing League unified in 2008. More precisely, CART’s successor, the Champ Car World Series, reached an agreement with the IRL in February 2008, bringing the two rival American open-wheel competitions together for the 2008 season.
The split had begun in the mid-1990s after Tony George created the IRL and reserved Indianapolis 500 places for its teams. CART continued as a rival championship, later becoming Champ Car after its 2003 bankruptcy. For 12 years, drivers and teams competed in separate series, weakening the sport and dividing its audience.
The 2008 agreement effectively ended Champ Car as a stand-alone series. The unified championship used the IRL’s existing IndyCar identity, equipment, and most of its schedule, while several Champ Car teams, drivers, and events joined the series. The arrangement is often called a merger, although the IRL acquired most of Champ Car’s assets. Thus, 2008—not 2005, 2010, or 2012—is the accepted answer.