How often must the operation of each airlock at the mine be tested?

The operation of each airlock at a mine must be tested at appropriate intervals to ensure that they function correctly and maintain safety standards. This approach allows for flexibility based on the specific conditions and hazards present at the mine, taking into account factors such as the frequency of use, operational changes, and any incidents that might affect the airlock's performance. Regular testing at intervals deemed appropriate by safety standards and risk assessments helps to mitigate the risk of failure and ensures compliance with safety regulations.

The other choices suggest fixed intervals or specific scenarios for testing, which may not consider the varying operational needs and conditions that a mine might face. Testing monthly may be unnecessarily frequent for some operations, while yearly testing could be inadequate for others where conditions change rapidly. Testing only after a malfunction disregards the proactive approach that is crucial in mining safety management.