Final answer:
ATIS messages must be retained for at least 30 days according to air traffic control regulations, providing a record that can be reviewed if needed.
Step-by-step explanation:
The retention period for Automatic Terminal Information Service (ATIS) messages is a regulatory requirement that ensures vital information remains available for a certain amount of time. In this case, option (c) 30 days is the correct answer. Air traffic control authorities require that ATIS messages, which contain important non-control information such as weather information, active runways, and other details relevant to pilots, be retained for at least 30 days. This allows for adequate record-keeping and review if necessary.