What lights indicate a system failure when the essential bus loses power?

The correct choice indicates that the EMER lights are utilized to signal a loss of power to the essential bus, which triggers a system failure alert. EMER lights are specifically designed to provide critical indications to the crew during abnormal situations, especially in cases of power loss. When the essential bus loses power, the EMER lights activate to inform the pilots of the failure, allowing them to take necessary actions to manage the aircraft's systems effectively.

Other options like warning lights, test lights, and CAS lights serve different functions and do not specifically indicate a system failure from the loss of power to the essential bus. While warning lights provide alerts for various system issues, test lights indicate the operational readiness of certain systems, and CAS lights are used for broader system alerting but not singularly for this specific failure condition. Understanding these distinctions is crucial for recognizing how the different lights play roles in the cockpit's overall alerting strategy.