The standard and primary method for removing infrastructure provisioned by Terraform is the terraform destroy command. This command reads the Terraform state file and issues API calls to the cloud provider to delete all managed resources in the correct dependency order.

However, for a stateful active-active HA cluster, simply destroying the underlying virtual machines can lead to unpredictable behavior. The FortiGate HA logic is designed to react to component failures. As Terraform deletes resources like network interfaces or instances, the cluster may interpret these actions as failures and trigger failover procedures. This can cause race conditions and API conflicts, potentially leading to a failed destroy operation. To ensure a clean and successful deletion, it is a best practice to first gracefully disable the HA configuration, effectively converting the cluster nodes into standalone devices before running terraform destroy.

B. Manually delete the FortiGate-A VM instance.

This is incorrect. Manually deleting resources managed by Terraform causes the state file to become inconsistent with the real-world infrastructure, leading to errors.

D. Manually delete the external and internal load balancers.

This is incorrect. Like other managed resources, load balancers should be deleted via Terraform to maintain state file consistency and ensure proper dependency resolution.

1. HashiCorp Terraform Documentation, destroy Command: "The terraform destroy command is a convenient way to destroy all remote objects managed by a particular Terraform configuration." This supports option A as the fundamental action for deletion. (Source: Terraform by HashiCorp, CLI Commands, destroy).

2. Fortinet Knowledge Base, Article ID: FD49400, "Technical Tip: How to gracefully shutdown a FortiGate HA cluster": This article outlines the procedure for a graceful shutdown, which begins with changing the HA mode to standalone on the primary unit. This prevents failover actions and ensures a clean shutdown. While written for general HA, the principle of disabling HA to prevent failover interference is directly applicable to ensuring a clean resource deletion in an automated context like Terraform. The first step is: "Change the mode to standalone on the primary FGT... config system ha, set mode standalone, end". This supports option C as a necessary preparatory step for a clean teardown.

3. FortiOS 7.4.1 Administration Guide, "High Availability" section: The guide explains that shutting down a primary unit causes the secondary to take over. This demonstrates that the HA system is active and will react to the removal of a node, which can interfere with an orchestrated deletion process. To avoid this, the cluster should be dismantled gracefully by disabling the HA configuration first. (Source: Fortinet Document Library, FortiOS 7.4.1 Administration Guide, p. 1985).