Which of the following is not a valid source from which Terraform can import modules?

Terraform modules can be sourced from various locations, providing flexibility in how you structure and manage your Infrastructure as Code (IaC). Among the listed options, the local filesystem, GitHub repository, and HTTP URL are standard and valid sources for importing modules due to their compatibility with Terraform's module sourcing mechanism.

Using the local filesystem allows users to reference modules that are stored on their own machines, making it straightforward to develop and test configurations locally. GitHub repositories are widely used as they enable sharing and versioning of Terraform code efficiently; Terraform can directly source modules from a specified repository URL. Additionally, HTTP URLs can be used to source modules hosted on web servers, provided the server hosts the appropriate files in an accessible format.

In contrast, while FTP servers can technically host files, Terraform does not support sourcing modules directly from them. The standard practice focuses on HTTP and similar protocols for seamless integration and compatibility, which is why sourcing from an FTP server is not considered a valid option in Terraform. Thus, this distinction underscores the nature of valid module sources within Terraform's ecosystem.