Final answer:
To automate the quarantine of a VPN device, FortiManager is the best option out of those listed because it offers centralized management and automation capabilities. FortiGate GUI and FortiAP are not designed for this purpose, while CLI would require manual commands.
Step-by-step explanation:
To quarantine a VPN device using automation, the most suitable options are likely a centralized management tool or the command line interface (CLI). Given the options FortiGate GUI, FortiManager, CLI, and FortiAP, the most appropriate choice would be b) FortiManager. FortiManager is designed to provide central management for multiple Fortinet devices and can automate tasks across your FortiGate installations.
By using FortiManager, you can create policies that automatically quarantine devices based on certain triggers, such as security alert thresholds. The FortiManager can push these policies to the connected FortiGate devices. While the CLI could also be used to perform such tasks, it requires manual input for each command and lacks the centralized, automated capabilities of FortiManager.
The FortiGate GUI and FortiAP are not designed for automation across multiple devices in the same way. The GUI is better suited for individual device configuration, and the FortiAP is an access point product that doesn't manage VPN device quarantine.