If you did not understand a radio transmission, what should you say?

When you did not understand a radio transmission, using "Say Again" is the appropriate response. This phrase is a clear and concise way to request that the sender repeat their message. It is widely recognized in maritime communication to indicate that the previous communication was not understood, thereby ensuring clarity and safety in operations.

The use of "Say Again" emphasizes the need for repetition without ambiguity and is in line with standard communication protocols. This approach helps to prevent misunderstandings that could have serious implications in a maritime context.

The other options do not convey the same clarity or intention. For example, "Confirm your message" suggests that you need validation of the information provided rather than a repeat, which may not resolve your lack of understanding. "Repeat" may not be a preferred term in official communication protocols. "Understood" implies that you have comprehended the original message, which contradicts the situation where clarification is needed. Therefore, "Say Again" is the most effective and precise choice for requesting repetition.