What type of radial should be used for Case III departures?

The correct choice for the type of radial to be used for Case III departures is the departure radial. In air traffic control, particularly during instrument flight rules (IFR) operations, a departure radial is specifically designated for aircraft that are taking off and transitioning into the en route phase of their flight. This radial provides a predetermined path that ensures safe and efficient routing away from the airport, minimizing the risk of conflicts with other traffic in the vicinity.

For Case III departures, which typically involve conditions where visibility is limited and dependence on instrument navigation is crucial, following a designated departure radial allows pilots to maintain situational awareness and adhere to the standardized flight path, ultimately enhancing safety during the departure phase.

The other options, such as arrival radials, are used for guiding aircraft towards an airport, horizontal radials do not have a defined standard use, and emergency radials are not a recognized term in the context of standard procedures for departures. Each of these alternatives serves different operational purposes that do not align with the requirements for safely managing departures under Case III conditions.