A is incorrect because the terraform plan command will not deploy any resources at all. It will only show the changes that would be made if the terraform apply command was run. The error message in the exhibit indicates that the service principal details are invalid, which means that Terraform cannot authenticate to Azure and cannot create any resources1.
B is incorrect because you can run the terraform apply command without running the terraform plan command first. The terraform apply command will automatically generate a new plan and prompt you to approve it before applying it2. However, running the terraform plan command first can help you preview the changes and avoid any unwanted or unexpected actions.
C is correct because you must run the terraform init command once before the terraform plan command.
The terraform init command initializes a working directory containing Terraform configuration files. It downloads and installs the provider plugins required for your configuration, such as the Azure provider2. It also creates a hidden directory called .terraform to store the plugin binaries and other metadata1. Without running the terraform init command, the terraform plan command will fail because it cannot find the required plugins or modules.
D is correct because the terraform plan command makes Terraform do a dry run. A dry run is a simulation of what would happen if you executed a certain action, without actually performing it. The terraform plan command creates an execution plan, which is a description of the actions that Terraform would take to make your infrastructure match your configuration2. The execution plan shows you what resources will be created, modified, or destroyed, and what attributes will be changed. The execution plan does not affect your infrastructure or state file until you apply it with the terraform apply command1.

The Azure client secret is a one-time value that is only visible when it is created. If the administrator loses or forgets the client secret, they cannot retrieve it from the Azure portal.
However, they can create a new client secret and use it to configure Terraform. To create a new client secret, they need to follow these steps:
Sign in to the Azure portal and navigate to the Azure Active Directory service. Select the application name under the App Registrations. Select Certificates & Secrets > New client secret to create a new client secret. Add a description and an expiration date for the client secret and select Add. Copy the value of the new client secret immediately as it will not be shown again.

Explanation
To deploy a FortiGate HA solution in AWS using Terraform, you need to create an AWS IAM user with permissions to access the AWS resources and services required by the FortiGate-VM. You also need to use CloudShell to install Terraform, which is a tool for building, changing, and versioning infrastructure as code.
References:
Deploying FortiGate-VM using Terraform | AWS Administration Guide
Setting up IAM roles | AWS Administration Guide
Launching the instance using roles and user data | AWS Administration Guide Terraform by HashiCorp