Which phrase would indicate to someone that a response is required?

The phrase "End of Message, Did You Receive?" clearly indicates that a response is required. This is because it not only signals that the message has concluded, but it also directly asks for confirmation of receipt. Using this phrase emphasizes the importance of feedback from the receiving party, which is crucial in communication, especially in scenarios where information accuracy and acknowledgment are vital, such as in law enforcement or emergency situations.

In contrast, the other phrases provide information or instructions but do not solicit a direct response. For instance, "Out of Service / At home" simply communicates status without requesting action. "Request Radio / Electronics Repair" indicates a need for maintenance but does not ask for a response. "Pick Up Papers at (location)" provides a directive but, again, lacks any prompt for acknowledgment or feedback. This distinction is essential for ensuring effective communication in high-stakes environments.