Which argument must be included in the module block?

The argument that must be included in the module block is the source. This argument specifies the location from which the module will be sourced. It can point to various types of sources, including a path to a local module, a Terraform Registry, a GitHub repository, or any other supported module source. Without this argument, Terraform would not know where to find the module's configuration files, which are necessary for the module to function correctly.

While other arguments play important roles in module configuration—for example, the version can specify which version of a module to use, and providers can define the provider configuration for the module—specifying the source is essential for the module's reference and instantiation. The depends_on argument provides a way to manage dependencies but is not strictly required to define a module block. Thus, the source is the foundational argument that must always be included in any module block to define its origin and make it usable.