Which log level should be used to indicate serious problems that require immediate attention?

The log level used to indicate serious problems requiring immediate attention is 'Critical.' This log level is designed for situations where the application's functionality is severely impacted or where a failure has occurred that could lead to data loss, system crashes, or other critical issues that must be addressed without delay. By categorizing logs as 'Critical,' developers and system administrators can quickly identify and prioritize resolutions to these serious incidents, ensuring the stability and reliability of the application.

The other log levels serve different purposes: 'Debug' is typically used for detailed debugging information that helps during development and troubleshooting. 'Warning' indicates a potential issue that may not require immediate action but should be monitored. 'Info' is used for general informational messages that track the normal operations of the application. Each of these levels has its use case, but none signify the urgency and severity implied by 'Critical.'