Which action must be taken to zeroise the KYV-5?

To effectively zeroise the KYV-5, it is essential to understand that the process involves multiple specific steps that ensure all sensitive cryptographic information is erased, thereby rendering the device back to its initial state. The action comprises observing a momentary flicker of the alarm, which is a critical indicator that the device is functioning correctly and that the zeroisation process can commence.

Additionally, the step of pulling down to unlock the ZERO ALL/INT Switch is necessary to engage the zeroisation function, as it prevents accidental activation. Finally, placing the switch into the ZEROALL (left) position initiates the actual zeroisation process. Each of these actions is crucial and interconnected, ensuring a complete reset of the device.

Thus, selecting the option that encapsulates all these necessary steps aligns with the correct procedure for zeroising the KYV-5, confirming the importance of each action in the overall process.