Explanation
The correct answer is B and D. A transit gateway with an attachment and a transit VPC support east-west traffic inspection within the AWS cloud by the FortiGate VM.
According to the Fortinet documentation for Public Cloud Security, a transit gateway is a network transit hub that connects VPCs and on-premises networks. A transit gateway attachment is a resource that connects a VPC or VPN to a transit gateway.By using a transit gateway with an attachment, you can route traffic from your spoke VPCs to your security VPC, where the FortiGate VM can inspect the traffic1.
A transit VPC is a VPC that serves as a global network transit center for connecting multiple VPCs, remote networks, and virtual private networks (VPNs).By using a transit VPC, you can deploy the FortiGate VM as a virtual appliance that provides network security and threat prevention for your VPCs2.
The other options are incorrect because:
A NAT gateway with an EIP is a service that enables instances in a private subnet to connect to the internet or other AWS services, but prevents the internet from initiating a connection with those instances.A NAT gateway with an EIP does not support east-west traffic inspection within the AWS cloud by the FortiGate VM3.
An Internet gateway with an EIP is a horizontally scaled, redundant, and highly available VPC component that allows communication between instances in your VPC and the internet.An Internet gateway with an EIP does not support east-west traffic inspection within the AWS cloud by the FortiGate VM4.
1:Fortinet Documentation Library - Deploying FortiGate VMs on AWS2: [Fortinet Documentation Library - Transit VPC on AWS]3: [NAT Gateways - Amazon Virtual Private Cloud]4: [Internet Gateways - Amazon Virtual Private Cloud]

Immutable infrastructure is a DevOps approach that emphasizes the creation of disposable resources instead of modifying existing ones1. This approach helps to achieve stability, consistency, and predictability in IT operations by reducing the risk of configuration drift and eliminating stateful components1.
One way to implement immutable infrastructure is to use a blue-green deployment strategy, which runs two live environments for configuration changes2. The blue environment is the current production environment, while the green environment is the new version of the application or service. When the green environment is ready, the traffic is switched from blue to green, and the blue environment is destroyed or kept as a backup2.
This way, there is no need to update or patch the existing infrastructure, but rather replace it with a new one.
References:
* 1: Immutable Infrastructure, Architecture, and its benefits
* 2: Introduction to Immutable Infrastructure - BMC Software | Blogs

In this scenario, the issue is caused by the Azure service principle account nothaving a contributor role. This is required for the FortiGate HA floating IP to work properly. Without this role, the new primary device will not have the previous primary device floating IP address after failover. References: Fortinet Public Cloud Security knowledge source documents or study guide.
https://docs.fortinet.com/product/fortigate-public-cloud/7.2

Confirming to delete all the resources in Terraform will have the following impact:
D:It destroys all the resources in the state file.
* Terraform State File Role:Theterraform.tfstatefile contains a real-time mapping of the resources that Terraform manages, including their current configuration and relationships. This file tracks the actual state of resources provisioned by Terraform.
* Impact of Destruction:When Terraform prompts for confirmation to destroy resources, and 'yes' is entered, Terraform reads the state file and systematically removes all the resources that are managed as part of that state. This is not limited to a specific .tfvars file, IAM user, or resource group-it is a global action that affects all resources tracked by the state file associated with the current Terraform workspace and configuration.
References:The function of theterraform.tfstatefile and the impact of resource destruction are detailed in Terraform's official documentation. This behavior is fundamental to how Terraform manages infrastructure as code.