Which response would indicate a unit is unable to respond to calls?

The response that indicates a unit is unable to respond to calls is "Out of service." When a unit is designated as out of service, it means that they are not available for assignments or responding to emergencies. This could be due to various reasons such as mechanical issues, officer breaks, or other operational factors that prevent the unit from being deployed effectively.

In contrast, the term "busy" would suggest that the unit is currently engaged in another call but may still be able to respond to new assignments once they are free. "In service" indicates that the unit is actively available and ready to respond to calls. "Relay" refers to the communication of messages or information but does not imply the status of availability for emergency response. Understanding these distinctions is essential for effective communication and operational management within law enforcement agencies.