Final answer:
The correct radio 10 code for requesting a repeat of a transmission is 10-9.
Step-by-step explanation:
The radio 10 code used when you don't understand what the dispatcher has said and you need them to repeat their transmission is 10-9. This code is specifically reserved for signaling that a message was not clear and a repeat is requested. In contrast, 10-4 is used to acknowledge that a message has been received, 10-78 typically indicates a request for assistance, and 10-99 can mean that a situation or signal is indicating danger or an officer needs assistance.