What should you say if you are unsure of the station calling?

Choosing "SAY AGAIN" is the appropriate response when you are unsure of the station calling because it directly requests the speaker to repeat their message. This phrase is clear and unambiguous, indicating that the listener did not fully understand or hear the original communication.

When you use "SAY AGAIN," you are effectively communicating that you are seeking clarification without any assumptions about the communication that was just made. This is vital in aviation communication, where clarity is essential for safety and operational efficiency.

In contrast, the other options might convey different nuances. "REPEAT" could imply that you are asking the station to repeat their last transmission, but it is less commonly used in aviation terminology. "REQUEST STAND BY" suggests that you need them to hold on while you gather your thoughts or respond, which may not be suitable if you simply did not hear them. "ACKNOWLEDGE" indicates that you have understood a previous message, which would not be appropriate in this context of uncertainty about who is calling.