What device is worn to signify completion of a training course and service?

The correct answer is the Arrowhead device. This device is awarded to personnel who have completed designated training courses and participated in certain operations, symbolizing a combination of training and service. Its distinctive shape and designation make it easily recognizable, indicating that the individual has not only completed a necessary training requirement but has also actively served in a specific context.

In contrast, clasps typically denote additional awards or achievements, service stars represent additional service in campaigns or specific periods, and the Berlin Airlift device signifies participation specifically in the Berlin Airlift operation. Therefore, while these other items may indicate service or achievement in certain contexts, the Arrowhead device is unique in its representation of both training completion and service in a specific operational capacity.