The correct syntax to pass a variable from the root module to a child module isservers = var.num_servers.
Terraform uses dot notation to reference variables.
References:
Terraform Variables

Terraform creates the .terraform.lock.hcl file after the first terraform init command. This lock file ensures that the dependencies for your project are consistent across different runs by locking the versions of the providers and modules used.

The "version" attribute is optional when referencing a module from the Terraform Registry. If not specified, the latest version will be used, but it is often recommended to specify a version to ensure consistency across environments.
References:
* Terraform Module Versioning

Option C is the correct way to configure multiple providers in a Terraform configuration. Each provider block must have a name attribute that specifies which provider it configures2. The other options are either missing the name attribute or using an invalid syntax.

Destroy Command Options: The terraform destroy command is the correct method to destroy resources, and it requires either manual approval or the -auto-approve flag.
Unsupported Triggering Method: The --destroy flag is not a recognized option for plan or apply commands, making B the correct answer as it is not a valid way to initiate resource destruction.
For more on destroying resources, refer to the terraform destroy command documentation in the Terraform CLI reference.