The Azure SDN connector uses two types of queries to interact with the Azure management API. The first query is targeted to a special IP address to get a token. This token is used to authenticate the subsequent queries. The second type of query is used to retrieve information about the Azure resources, such as virtual machines, network interfaces, network security groups, and public IP addresses. Some queries are made to manage public IP addresses, such as assigning or releasing them from the FortiGate VM. Reference: Configuring an SDN connector in Azure, Azure SDN connector using service principal, Troubleshooting Azure SDN connector

When an administrator decides to use the 'Use managed identity' option for the FortiGate SDN connector with Microsoft Azure and faces a connection failure, the correct action to take is: C.
Make sure to enable the system assigned managed identity on Azure.
Managed Identity Configuration: The system assigned managed identity is a feature in Azure that provides an identity for the Azure service instance (in this case, the FortiGate SDN connector) within Azure Active Directory and eliminates the need for credentials to be stored in the configuration.
Troubleshooting Connection Issues: If the SDN connector is failing to connect, it could be because the system assigned managed identity has not been enabled or configured properly in Azure for the FortiGate service.

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 tunnels. 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 tunnels.

Explanation
According to the AWS documentation for Transit Gateway, a transit gateway is a network transit hub that connects VPCs and on-premises networks. A transit gateway route table is a set of rules that determines how traffic is routed among the attachments to the transit gateway1.
A transit gateway can have multiple route tables, and you can associate different attachments with different route tables. This allows you to control how traffic is routed between your VPCs and VPNs based on your network design and security requirements1.
The other options are incorrect because:
Both the TGW attachment and propagation must be in the same TGW route table is not true. You can associate an attachment with one route table and enable propagation from another attachment to a different route table. This allows you to separate the routing domains for your attachments1.
A TGW attachment can be associated with multiple TGW route tables is not true. You can only associate an attachment with one route table at a time. However, you can change the association at any time1.
The TGW default route table cannot be disabled is not true. You can disable the default route table by deleting all associations and propagations from it. However, you cannot delete the default route table itself1.
1: Transit Gateways - Amazon Virtual Private Cloud

Explanation
The error message indicates that the Connect attachment is already associated with another transit gateway route table. You cannot associate the same attachment with more than one route table. However, you can propagate the same attachment to multiple route tables. Therefore, to fulfill your requirement of configuring a second route table for east-west traffic inspection between two VPCs, you need to create a propagation with the Connect attachment in the second route table. This will allow the second route table to learn the routes from the Connect attachment and forward the traffic to the securityVPC1. You also need to associate the second route table with the Transport attachment, which is the transit gateway attachment for the security VPC1.
References:
Transit gateway route tables - Amazon VPC | AWS Documentation
Getting started with transit gateways - Amazon VPC | AWS Documentation
Configuring TGW route tables | FortiGate Public Cloud 7.4.0 | Fortinet Document Library