Which term indicates that the transmission was understood or accepted?

The term indicating that the transmission was understood or accepted is "Acknowledgement / OK." This terminology is crucial in communications, especially in law enforcement and emergency services, as it confirms that a message has been received and properly understood by the recipient. When a person acknowledges a transmission, it ensures clear and effective communication, reducing the likelihood of misunderstandings or errors in operational contexts.

In contrast, the other terms do not imply acceptance or understanding of a communication. "Relay" refers to passing a message from one party to another without confirming its understanding. "Out of Service" indicates that a unit or officer is not currently operational, while "In Service" means that a unit or officer is actively responding and ready for calls. Neither of these options conveys the idea that a message was received or comprehended, which makes "Acknowledgement / OK" the most appropriate choice.