What command initializes a Terraform working directory?

Correct answer: terraform init

The command 'terraform init' initializes a Terraform working directory by downloading provider plugins, setting up the backend, and preparing modules declared in the configuration files. Option A is incorrect because 'terraform configure' is not a valid Terraform CLI command. Option C is incorrect because 'terraform start' does not exist in the Terraform CLI. Option D is incorrect because 'terraform setup' is also not a valid Terraform command; the correct initialization command has always been 'terraform init'.