Which of these options is the most secure place to store secrets for connecting to a Terraform remote backend?

Storing secrets in environment variables is considered a secure practice because it keeps sensitive information out of the codebase, reducing the risk of accidental exposure. Using environment variables allows you to manage sensitive data separately from the Terraform configuration files, meaning that secrets are not written in plaintext where they could be viewed by anyone with access to the repository.

When you use environment variables, you can set them at the operating system level or in a secure context such as a CI/CD pipeline. This method enhances security because the secrets are not hardcoded into the configuration or stored in files that could be committed to version control systems. Additionally, managing secrets through environment variables aligns with best practices for secret management, allowing for better control over access and the possibility of leveraging secret management tools.

Other options, like storing secrets in plaintext files or hardcoding them directly in scripts, expose secrets to anyone who can access the files or code, making them susceptible to leaks. Similarly, sharing secrets in public repositories poses a significant security risk, as it allows anyone to view the sensitive information. Hence, utilizing environment variables is the most secure method among the options given.