Which option cannot be used to keep secrets out of Terraform configuration files?

Using secure strings to keep secrets out of Terraform configuration files refers to an approach that may not be universally supported in all contexts. It may imply an idea but is not a specific feature of Terraform like the other options. In contrast, environment variables are commonly used to inject sensitive data such as API keys without hardcoding them into configuration files. Terraform variables files allow users to define sensitive variables, which can be loaded with a command, but they still require careful management to ensure the files themselves do not inadvertently expose sensitive information.

Remote state files, when configured properly, can store state securely and can help manage sensitive data from being directly embedded in configurations. However, secure strings as a standalone method may lack the robust support or consistency found in Terraform's established methods for managing secrets effectively. Therefore, it stands out as the least actionable option specifically related to keeping secrets away from configuration files.