The correct answer is C and D. Use a transit VPC with hub and spoke topology to create multiple VPN connections to the on-premises data center. Use the transit gateway attachment with VPN option to create multiple VPN connections to the on-premises data center.
According to the Fortinet documentation for Public Cloud Security, 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). A transit VPC can use a hub and spoke topology to create multiple VPN connections to the on-premises data center, using the FortiGate VM as a virtual appliance that provides network security and threat prevention.A transit VPC can also leverage Equal-Cost Multi-Path (ECMP) routing to achieve higher bandwidth and load balancing across multiple VPN tunnels1.
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. You can use the transit gateway attachment with VPN option to create multiple VPN connections to the on-premises data center, using the FortiGate VM as a virtual appliance that provides network security and threat prevention.A transit gateway attachment with VPN option can also leverage ECMP routing to achieve higher bandwidth and load balancing across multiple VPN tunnels2.
The other options are incorrect because:
* Using ECMP and VPN to achieve higher bandwidth is not a complete solution, as it does not specify how to replace the existing VPC peering topology or how to connect the AWS VPCs to the on-premises data center.
* Using transit VPC to build multiple VPC connections to the on-premises data center is not a correct solution, as it does not specify how to use a hub and spoke topology or how to leverage ECMP routing for higher bandwidth.
1:Fortinet Documentation Library - Transit VPC on AWS2:Fortinet Documentation Library - Deploying FortiGate VMs on AWS

The variables.tf file shows that the FortiGate VM will be deployed in the eu-West-Ia region with private and public subnets. The region variable is set to "eu-west-1" and the availability_zone variable is set to
"eu-west-1a". The vpc_id variable is set to "vpc-0e9d6a6f" and the subnets variable is set to a list of two subnet IDs: "subnet-0f9d6a6f" and "subnet-1f9d6a6f". The license_type variable is set to "on-demand" and the ami_id variable is set to "ami-0e9d6a6f".
References:
https://docs.fortinet.com/document/fortigate/6.4.0/aws-cookbook/236478/deploying-fortigate-vm-on-aws-using-t

Terraform manages and tracks the state of infrastructure resources through a file known asterraform.tfstate.
This file is automatically created by Terraform and is updated after the application of a Terraform plan to capture the current state of the resources.
* State File Purpose:Theterraform.tfstatefile contains a JSON object that records the IDs and properties of resources Terraform manages, so that it can map real-world resources to your configuration, keep track of metadata, and improve performance for large infrastructures.
* State File Management:This file is crucial for Terraform to perform resource updates, deletions, and for creating dependencies. It's essentially the 'source of truth' for Terraform about your managed infrastructure and services.
References:This behavior is documented in Terraform's official documentation, which explains how the terraform.tfstatefile is used to keep track of the infrastructure Terraform is managing.

Explanation
B: Add route destination 0.0.0.0/0 to target the transit gateway. This will ensure that the Customer VPC FortiGate VM sends all the outbound internet traffic through the Security VPC, where it can be inspected by the Security VPC FortiGate VMs1. The transit gateway is a network device that connects multiple VPCs and on-premises networks in a hub-and-spoke model2. D. Deploy an internet gateway, associate an EIP in the private subnet, edit route tables, and add a new route destination 0.0.0.0/0 to the target internet gateway. This will allow inbound HTTPS access to the Customer VPC FortiGate VM from the internet, by creating a public route for the private subnet where the FortiGate VM is located3. An internet gateway is a service that enables communication between your VPC and the internet4. An EIP is a public IPv4 address that you can allocate to your AWS account and associate with your resources. E. Deploy an internet gateway, associate an EIP in the public subnet, and attach the internet gateway to the Customer VPC. This will also allow inbound HTTPS access to the Customer VPC FortiGate VM from the internet, by creating a public route for the public subnet where the FortiGate VM is located3. This is an alternative solution to option D, depending on which subnet you want to use for the FortiGate VM.
The other options are incorrect because:
Adding a route with your local internet public IP address as the destination and target transit gateway will not allow inbound HTTPS access to the Customer VPC FortiGate VM from the internet, because it will only apply to traffic coming from your specific IP address, not from any other source on the internet1. Moreover, it will not ensure that the outbound internet traffic goes through the Security VPC, because it will only apply to traffic going to your specific IP address, not to any other destination on the internet1.
Adding a route with your local internet public IP address as the destination and target internet gateway will not allow inbound HTTPS access to the Customer VPC FortiGate VM from the internet, because it will bypass the Security VPC and send the traffic directly to the Customer VPC1. Moreover, it will not ensure that the outbound internet traffic goes through the Security VPC, because it will only apply to traffic going to your specific IP address, not to any other destination on the internet1.