Which of the following is a possible indication of ELT transmission?

One possible indication of an Emergency Locator Transmitter (ELT) transmission is a "CAS message ELT ON and external buzzer." This answer holds validity because when an ELT is activated, it often triggers a cockpit alert through a CAS (Crew Alerting System) message, notifying the flight crew that the device is on. Additionally, an external buzzer sound serves as an acoustic alert, further indicating that the ELT has been engaged. This dual notification system ensures that the crew is promptly informed of any situation that might require their attention regarding the aircraft's emergency status.

The other options may not effectively signify an ELT activation. While a visual alert on the Multi-Function Display (MFD) could indicate various issues, it does not specifically confirm that the ELT is transmitting. Flashing lights on the dashboard might be associated with other malfunction alerts, and a cabin intercom announcement, while potentially important, is not a direct indicator of an ELT transmission. Thus, the specific combination of the CAS message and external buzzer clearly points to an ELT activation.