What command is used to initialize a Terraform working directory?

The command used to initialize a Terraform working directory is "terraform init." This command is crucial because it prepares the directory for other Terraform commands by setting up the necessary files and downloading required provider plugins. When you run "terraform init," Terraform performs several important tasks, such as:

1. Initializing Backend: If you are using a remote backend for state management, this command configures the backend.
2. Installing Provider Plugins: Terraform retrieves and installs the provider plugins specified in your Terraform configuration files. These plugins are essential for Terraform to interact with the cloud services or APIs defined in your infrastructure code.
3. Creating a .terraform Directory: This command creates a hidden directory called .terraform within your working directory, which stores various files related to the initialized state and downloaded plugins.

The other options provided do not correspond to any valid Terraform operations. "terraform start," "terraform bootstrap," and "terraform setup" are not recognized commands within the Terraform CLI, which is why they would not be the correct choice for initializing a working directory.