Which terminals at O'Hare are primarily used for domestic flights?

Terminals 1, 2, and 3 at O'Hare Airport are primarily designated for domestic flights. Terminal 1 primarily serves United Airlines and some regional carriers, while Terminal 2 hosts a mix of airlines, including Delta Air Lines, and terminal operations for some regional services. Terminal 3, on the other hand, is largely used by American Airlines and other domestic carriers.

This configuration allows for efficient handling of domestic air travel within the airport, as these terminals are strategically located to support domestic operations with their infrastructure, check-in counters, and security processes geared toward that segment of travel.

The distinction of these terminals as primarily domestic contrasts with other terminals, such as Terminal 5, which is focused on international flights, thus explaining why the other choices do not include the correct grouping of terminals for domestic services.