What action must a cabin crew member take if they find a note on board?

The appropriate action for a cabin crew member upon discovering a note on board is to immediately report to the captain. This is essential because the captain is responsible for the overall safety and security of the aircraft and its passengers. The discovery of a note could potentially indicate a security threat, a passenger distress signal, or other important information that necessitates prompt attention.

In an aviation context, the captain must be made aware of any irregularities or findings that could impact flight safety. By communicating directly with the captain, the crew member ensures that the information is evaluated quickly and appropriately, allowing for any necessary measures to be taken, such as alerting authorities or implementing security protocols.

The other options do not prioritize communication with the captain in the way that is vital for ensuring immediate and proper action can be taken. Discarding the note would lead to a loss of potentially critical information, while merely reporting to another crew member or the commander may delay the response time and might not convey the urgency of the situation.