If a message is received clearly, which code could be used to confirm?

The code that confirms a message has been received clearly is "10-4." This code is part of the ten-codes used in radio communications, which serve to communicate information succinctly and effectively. Specifically, "10-4" translates to "Acknowledged" or "Message received." When someone transmits information and the recipient wants to indicate that they understand it clearly, using "10-4" is the appropriate acknowledgment.

The other choices represent different communications meanings: "10-7" generally indicates "Out of service," "10-10" means "Transmission completed," and "10-21" refers to making a phone call. These codes serve specific purposes but do not convey the clear acknowledgment of message receipt as "10-4" does. Therefore, using "10-4" effectively communicates that the message has been understood without ambiguity.